[BUG]: Extension method JsonSerDe SortProperties removes arrays with strings from metadata during serialization

[TrondRo]

Describe the bug
If you include an array of values in a StructField metadata instance they will not serialize when .Json is called on the root StructType. They will however show on the fields JsonValue.

To Reproduce

Steps to reproduce the behavior:

var values = new[] { "Test1", "Test2" };
var metadata = new JObject(new JProperty("allowedValues", values));
var field = new StructField("TestField", new StringType(), true, metadata);
var fields = new StructType(new[] { field });
var json = fields.Json;

json will contain {"fields":[{"metadata":{"allowedValues":[]},"name":"TestField","nullable":true,"type":"string"}],"type":"struct"}

Expected behavior
I would expect allowedValues in the example above to contain Test1 and Test2.

Screenshots
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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]
  Microsoft.Spark 2.1.1

Additional context
The code that sorts the array in the serialization when you call DataType.Json does not take into account if the item is a simple string and thus returns an empty array:

spark/src/csharp/Microsoft.Spark/Interop/Ipc/JsonSerDe.cs

Lines 55 to 65 in 45d0058

	foreach (JToken item in jArray)
	{
	if (item is JObject elem)
	{
	sortedJArray.Add(elem.SortProperties());
	}
	else if (item is JArray arrayElem)
	{
	sortedJArray.Add(arrayElem.SortProperties());
	}
	}