Az.Synapse Set-AzSynapsePipeline - Ignores literal nulls (value = null) in json definition

### Description

This module has a bug when using the Set- cmdlets to publish json definition objects to Synapse.

If the JSON source file contains elements specified as literal nulls when the definition is published to Synapse it ignores the JSON elements that were literal nulls (i.e. value = null). This causes the use of the 'Treat as Null' Stored Procedure activity to not be publishable from the json files. I was able to work around this via directly using the Synapse Rest APIs, the same files published via the APIs do not strip away any literal null json element.

### Issue script & Debug output

```PowerShell
n/a
```

### Environment data

```PowerShell
5.1
```

### Module versions

```PowerShell
I tested via versions 0.1.0, 0.7.0, 0.18.0, the bug occurs with all versions
```

### Error output

```PowerShell
No error output from Powershell, the bug is that the published result to Synapse will ignore any json element set to equal a literal null (i.e. value = null)
```