[SDK-396] Parse correctly empty string

[nzagorchev]

What	Where/Who
JIRA Issue	SDK-396

Background

If Decryption fails, String.Empty is returned. This cannot be parsed by MiniJSON and throws an exception.
Such exception can be thrown in VarCache.LoadDiffs when calling ApplyVariableDiffs.

Implementation

Return null for String.Empty when parsing JSON.

Testing steps

var test = LeanplumSDK.MiniJSON.Json.Deserialize("");

The above line was previously throwing an exception:

OverflowException: Value was either too large or too small for a character.
at System.Convert.ToChar (System.Int32 value)

It now returns null.

Other cases remain unchanged:

var test = LeanplumSDK.MiniJSON.Json.Deserialize(" "); // returns null
var test = LeanplumSDK.MiniJSON.Json.Deserialize("  "); // returns null

Is this change backwards-compatible?

Yes