"az network dns zone export" fails with empty TXT records

### Describe the bug

Attempting to export a DNS zone with a TXT record that has an empty value results in the following error:

```
The command failed with an unexpected error. Here is the traceback:
'txt'
Traceback (most recent call last):
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\knack/cli.py", line 233, in invoke
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/cli/core/commands/__init__.py", line 663, in execute
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/cli/core/commands/__init__.py", line 726, in _run_jobs_serially
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/cli/core/commands/__init__.py", line 697, in _run_job
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/cli/core/commands/__init__.py", line 333, in __call__
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/cli/core/commands/command_operation.py", line 121, in handler
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/cli/command_modules/network/custom.py", line 2633, in export_zone
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/cli/command_modules/network/zone_file/make_zone_file.py", line 96, in make_zone_file
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/cli/command_modules/network/zone_file/record_processors.py", line 134, in process_txt
File "D:\a\_work\1\s\build_scripts\windows\artifacts\cli\Lib\site-packages\azure/cli/command_modules/network/zone_file/record_processors.py", line 55, in _quote_field
KeyError: 'txt'
To check existing issues, please visit: https://github.com/Azure/azure-cli/issues
```

### Related command

```
az network dns zone export
--subscription "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
--resource-group "my-dns"
--name "my.dns.zone"
```

### Errors

See error in description above.

### Issue script & Debug output

**Set up a test environment**

This script sets up a new dns zone with an empty TXT record so we can reproduce the issue, but you can create the zone and empty TXT record manually in a browser via https://portal.azure.com as well...

```powershell
# set up some variables for the script
$subscriptionId = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx";
$resourceGroupName = "my-dns";
$zoneName = "my.dns.zone";
$recordType = "TXT";
$relativeRecordSetName = "@";
$apiVersion = "2023-07-01-preview";

# create a new dns zone so we've got a clean environment to test
$azZone = New-AzDnsZone `
-ResourceGroupName $resourceGroupName `
-Name $zoneName;

# add a TXT recordset with an empty value
#
# note that neither "az cli" nor the Az PowerShell cmdlets will let us create
# a TXT record with an empty value - they both throw parameter validation
# errors, so we have to either create it manually in a browser at
# https://portal.azure.com or invoke the raw rest api using "az rest" which
# *do* allow empty values.
#
# see:
# https://learn.microsoft.com/en-us/cli/azure/reference-index?view=azure-cli-latest#az-rest
# https://learn.microsoft.com/en-us/rest/api/dns/record-sets/create-or-update?tabs=HTTP#create-txt-recordset

$resourceId = `
"/subscriptions/$subscriptionId" +
"/resourceGroups/$resourceGroupName" +
"/providers/Microsoft.Network" +
"/dnsZones/$zoneName" +
"/$recordType/$relativeRecordSetName";

# using Chrome's F12 dev tools shows this body is what https://portal.azure.com sends
# when creating an empty TXT record manually in a browser
$restBody = @{
"id" = $resourceId
"name" = "@"
"type" = "Microsoft.Network/dnszones/TXT"
"properties" = @{
"TTL" = 3600
"targetResource" = $null
"ARecords" = $null
"AAAARecords" = $null
"caaRecords" = $null
"CNAMERecord" = $null
"MXRecords" = $null
"NSRecords" = $null
"SOARecord" = $null
"SRVRecords" = $null
"TXTRecords" = $null
"PTRRecords" = $null
}
} | ConvertTo-Json -Depth 99 -Compress;
$restBody = $restBody.Replace('"', '\"');

az rest `
--uri "https://management.azure.com$($resourceId)?api-version=$apiVersion" `
--method "PUT" `
--headers "Content-Type=application/json" `
--body $restBody `
--debug
```

**Result of setup script**

The setup script creates a TXT record with an empty value - the portal renders a hyphen ```-``` to indicate an empty value.


**Reproduce the issue**

With the environment set up with an empty TXT record using the above script, this command reproduces the error:

```powershell
az network dns zone export `
--subscription $subscriptionId `
--resource-group $resourceGroupName `
--name $zoneName
```

### Expected behavior

The DNS zone gets serialized properly to text, *including* an empty TXT record.

### Environment Summary

```
azure-cli 2.52.0

core 2.52.0
telemetry 1.1.0

Dependencies:
msal 1.24.0b1
azure-mgmt-resource 23.1.0b2

Python location 'C:\Program Files\Microsoft SDKs\Azure\CLI2\python.exe'
Extensions directory '********'

Python (Windows) 3.10.10 (tags/v3.10.10:aad5f6a, Feb 7 2023, 17:20:36) [MSC v.1929 64 bit (AMD64)]

Legal docs and information: aka.ms/AzureCliLegal

Your CLI is up-to-date.
```

### Additional context

There are other duplicate issues reported but have been closed with workarounds that I can't use:

* https://github.com/Azure/azure-cli/issues/23645
* https://github.com/Azure/azure-cli/issues/27021

The suggested resolution for these was to delete the empty TXT record, but I don't have this option as I don't have jurisdiction over the DNS configuration

And a similar issue:

* https://github.com/Azure/azure-cli/issues/27391 - coincidentally raised a very short time ago. Affects AAAA and CNAME records, but It's probably got the same root cause based around handling empty values for fields