Fix VB -> C# char comparison with empty string

[GrahamTheCoder]

Fixes #892 by handling the equality comparison of a Char type and an empty string literal "".

In VB.NET, testChar = "" evaluates to True if testChar is char.MinValue. Before this change, the code converter converted this condition into Conversions.ToString(testChar) == "", which evaluates differently.
This patch maps such comparisons to testChar == char.MinValue or char.MinValue == testChar by ensuring that "" translates to char.MinValue when the other argument is of type System_Char.

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PR created automatically by Jules for task 2639555887942549494 started by @GrahamTheCoder

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