feat(common): support null value for reference type collection elements

[archerzz]

• update SerializationBuilder to set ValueSerialization.IsNullable : true for collection elements of reference type
  • that will cause null value handling in serialization & deserialization logic to be added for collection elements of reference type
• update SerializationWriter so that in generated model deserialization methods, null value handling (return null if pass in null json value) is added at the beginning
• update Utf8JsonWriterExtension.WriteBase64StringValue() to handle null json value in serialization
• update JsonElementExtension.GetBytesFromBase64() to handle null json value in deserialization

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update JsonCodeWriterExtension, for collection items:

• In list serialization, instead of generating null value handling logic for all nullable value serialization, only generating the logic for nullable value types. That can eliminate a lot null handling codes in serialization method. And for those without such logic:
  • string type can be handled by Utf8JsonWriter.WriteStringValue() which has null value support
  • byte[] is handled by the modified Utf8JsonWriterExtension.WriteBase64StringValue() (see the change above)
  • model classes and other reference types are handled by Utf8JsonWriterExtension.WriteObjectValue() which has null value support
• In dictionary serialization, add the missing null value handling for values
• In list & dictionary deserialization, change the generated codes to:
  • model class null value check is handled by their own deserialization method (see the change above)
  • string null value check in handled by JsonElement.GetString()
  • byte[] is handled by JsonElementExtension.GetBytesFromBase64()
  • nullable value type, list/dictionary, other framework reference will have generated null value check codes

---

• fix 3 test cases about null value in collection elements in deserialization

resolve #3159

Serialization

Serialization	Null check in	example	comments
nullable value type	generated codes in parent serialization method		if (value==null) { writer.WriteNullValue()}
SDK model	Utf8JsonWriterExtensions.WriteObjectValue()
string	Utf8JsonWriterExtensions.WriteStringValue()
byte[]	Utf8JsonWriterExtensions.WriteBase64StringValue()		need to add null check in WriteBase64StringValue
array, dictionary	generated codes in parent serialization method		if (value==null) { writer.WriteNullValue()}
other reference type	generated codes in parent serialization method		if (value==null) { writer.WriteNullValue()}

Deserialization

Serialization	Null Check in	example	comments
nullable value type	generated codes in parent deserialization method	int?	if (element.Value.ValueKind == JsonValueKind.Null) { array.Add(null)}
SDK model	model deserialization method	IPAddress	need to add null check in deserialization method
string	JsonElement.GetString()
byte[]	JsonElementExtension.GetBytesFromBase64()		need to add null check in GetBytesFromBase64
array, dictionary	generated codes in parent deserialization method		if (element.Value.ValueKind == JsonValueKind.Null) { array.Add(null)}
other framework reference type	generated codes in parent deserialization method		if (element.Value.ValueKind == JsonValueKind.Null) { array.Add(null)}

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[m-nash]

I believe we want to get your parallel work in before uploading the regen PR? If so lets mark this as draft until we get ready for that.

[archerzz]

Here is the generated preview PR: Azure/azure-sdk-for-net#34624