[Bug]: Cannot start DAB when a table has a non-supported data type, even if the column has been added to the exclude list

### What happened?

My table is the following:

```sql
create table vehicle_position_data
(
id varchar(20) not null primary key clustered,
trip_id int not null,
direction_id int not null,
vehicle_id varchar(20) not null,
route_id int not null,
latitude decimal (9,6) not null,
longitude decimal (9,6) not null,
[position] as geography::Point(latitude, longitude, 4326) persisted,
[timestamp] int not null,
stop_info nvarchar(max) not null check(isjson(stop_info)=1)
)
go
```

The `geography` data type is not yet supported, so I added the `position` column to the excluded columns:

```json
"entities": {
"Positions": {
"source": "dbo.vehicle_position_data",
"permissions": [
{
"role": "anonymous",
"actions": [
{
"action": "*",
"fields": {
"exclude": [ "position" ]
}
}
]
}
],
"rest": {
"path": "/positions"
}
}
}
```

But I get the error:

```
Unable to complete runtime initialization. Refer to exception for error details.
Azure.DataApiBuilder.Service.Exceptions.DataApiBuilderException: Cannot obtain Schema for entity Positions with underlying database object source: dbo.vehicle_position_data due to: DataReader.GetFieldType(7) returned null.
at Azure.DataApiBuilder.Service.Services.SqlMetadataProvider`3.GetTableWithSchemaFromDataSetAsync(String entityName, String schemaName, String tableName)
at Azure.DataApiBuilder.Service.Services.SqlMetadataProvider`3.PopulateSourceDefinitionAsync(String entityName, String schemaName, String tableName, SourceDefinition sourceDefinition, String[] runtimeConfigKeyFields)
at Azure.DataApiBuilder.Service.Services.SqlMetadataProvider`3.PopulateObjectDefinitionForEntities()
at Azure.DataApiBuilder.Service.Services.SqlMetadataProvider`3.InitializeAsync()
at Azure.DataApiBuilder.Service.Startup.PerformOnConfigChangeAsync(IApplicationBuilder app)
```

Which is not expected as the column has been excluded.

### Version

0.7.5

### What database are you using?

Azure SQL

### What hosting model are you using?

Local (including CLI)

### Which API approach are you accessing DAB through?

REST

### Relevant log output

```Text
Information: Microsoft.DataApiBuilder 0.7.5+2f938a6dfe4b90c26d9e24352b38280cb6e66f15
Found config file: dab-config.development.json.
Found config file: dab-config.json.
Information: Using merged config file based on environment:dab-config.development.merged.json.
Information: User provided config file: dab-config.development.merged.json
Information: Setting default minimum LogLevel: Debug for Development mode.
Starting the runtime engine...
info: Azure.DataApiBuilder.Service.Configurations.RuntimeConfigProvider[0]
Using file dab-config.development.merged.json to configure the runtime.
info: Azure.DataApiBuilder.Service.Configurations.RuntimeConfigProvider[0]
GraphQL type for Positions is Positions
info: Azure.DataApiBuilder.Service.Configurations.RuntimeConfigProvider[0]
Runtime configuration has been successfully loaded.
info: Azure.DataApiBuilder.Service.Configurations.RuntimeConfigProvider[0]
GraphQL path: /graphql
info: Azure.DataApiBuilder.Service.Configurations.RuntimeConfigProvider[0]
StaticWebApps
info: Azure.DataApiBuilder.Service.Configurations.RuntimeConfigProvider[0]
Runtime config loaded from file.
info: Microsoft.AspNetCore.DataProtection.KeyManagement.XmlKeyManager[63]
User profile is available. Using 'C:\Users\damauri\AppData\Local\ASP.NET\DataProtection-Keys' as key repository and Windows DPAPI to encrypt keys at rest.
info: Azure.DataApiBuilder.Service.Services.ISqlMetadataProvider[0]
Positions path: /api/Positions
fail: Azure.DataApiBuilder.Service.Startup[0]
Unable to complete runtime initialization. Refer to exception for error details.
Azure.DataApiBuilder.Service.Exceptions.DataApiBuilderException: Cannot obtain Schema for entity Positions with underlying database object source: dbo.vehicle_position_data due to: DataReader.GetFieldType(7) returned null.
at Azure.DataApiBuilder.Service.Services.SqlMetadataProvider`3.GetTableWithSchemaFromDataSetAsync(String entityName, String schemaName, String tableName)
at Azure.DataApiBuilder.Service.Services.SqlMetadataProvider`3.PopulateSourceDefinitionAsync(String entityName, String schemaName, String tableName, SourceDefinition sourceDefinition, String[] runtimeConfigKeyFields)
at Azure.DataApiBuilder.Service.Services.SqlMetadataProvider`3.PopulateObjectDefinitionForEntities()
at Azure.DataApiBuilder.Service.Services.SqlMetadataProvider`3.InitializeAsync()
at Azure.DataApiBuilder.Service.Startup.PerformOnConfigChangeAsync(IApplicationBuilder app)
info: Azure.DataApiBuilder.Service.Startup[0]
Loading config file: dab-config.development.merged.json
fail: Azure.DataApiBuilder.Service.Startup[0]
Exiting the runtime engine...
crit: Microsoft.AspNetCore.Hosting.Diagnostics[6]
Application startup exception
System.ApplicationException: Could not initialize the engine with the runtime config file: dab-config.development.merged.json
at Azure.DataApiBuilder.Service.Startup.Configure(IApplicationBuilder app, IWebHostEnvironment env, RuntimeConfigProvider runtimeConfigProvider)
at System.RuntimeMethodHandle.InvokeMethod(Object target, Span`1& arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at Microsoft.AspNetCore.Hosting.ConfigureBuilder.Invoke(Object instance, IApplicationBuilder builder)
at Microsoft.AspNetCore.Hosting.ConfigureBuilder.<>c__DisplayClass4_0.b__0(IApplicationBuilder builder)
at Microsoft.AspNetCore.Hosting.GenericWebHostBuilder.<>c__DisplayClass15_0.b__1(IApplicationBuilder app)
at Microsoft.AspNetCore.Mvc.Filters.MiddlewareFilterBuilderStartupFilter.<>c__DisplayClass0_0.g__MiddlewareFilterBuilder|0(IApplicationBuilder builder)
at Microsoft.AspNetCore.HostFilteringStartupFilter.<>c__DisplayClass0_0.b__0(IApplicationBuilder app)
at Microsoft.AspNetCore.Hosting.GenericWebHostService.StartAsync(CancellationToken cancellationToken)
Unable to launch the runtime due to: System.ApplicationException: Could not initialize the engine with the runtime config file: dab-config.development.merged.json
at Azure.DataApiBuilder.Service.Startup.Configure(IApplicationBuilder app, IWebHostEnvironment env, RuntimeConfigProvider runtimeConfigProvider)
at System.RuntimeMethodHandle.InvokeMethod(Object target, Span`1& arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
at Microsoft.AspNetCore.Hosting.ConfigureBuilder.Invoke(Object instance, IApplicationBuilder builder)
at Microsoft.AspNetCore.Hosting.ConfigureBuilder.<>c__DisplayClass4_0.b__0(IApplicationBuilder builder)
at Microsoft.AspNetCore.Hosting.GenericWebHostBuilder.<>c__DisplayClass15_0.b__1(IApplicationBuilder app)
at Microsoft.AspNetCore.Mvc.Filters.MiddlewareFilterBuilderStartupFilter.<>c__DisplayClass0_0.g__MiddlewareFilterBuilder|0(IApplicationBuilder builder)
at Microsoft.AspNetCore.HostFilteringStartupFilter.<>c__DisplayClass0_0.b__0(IApplicationBuilder app)
at Microsoft.AspNetCore.Hosting.GenericWebHostService.StartAsync(CancellationToken cancellationToken)
at Microsoft.Extensions.Hosting.Internal.Host.StartAsync(CancellationToken cancellationToken)
at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.RunAsync(IHost host, CancellationToken token)
at Microsoft.Extensions.Hosting.HostingAbstractionsHostExtensions.Run(IHost host)
at Azure.DataApiBuilder.Service.Program.StartEngine(String[] args)
Error: Failed to start the engine.
```

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