fix MultiByte <-> WideChar conversion return value on linux

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MultiByteToWideChar/WideCharToMultiByte are supposed to include/exclude null termination based on whether the source string buffer is null terminated.

This was done by comparing the input byte/wide character count to the output byte/wide character count excluding null termination. This only works correctly if the input only consists of ASCII characters. (ie. only characters that are single byte in UTF-8)

This commit changes the code to explicitly check for a null terminator in the input and add one to the output size if the input was terminated.

[exoticorn]

@microsoft-github-policy-service agree company="Keen Games GmbH"

[exoticorn]

Without this fix, I ran into the assert

    DXASSERT(
        !utf8CharCount ||
            IsBufferNullTerminated(utf8NewCopy.m_pData, utf8CharCount, CP_UTF8),
        "otherwise, CodePageBufferToUtf8 failed to null-terminate buffer.");

in DxcGetBlobAsUtf8 when loading a shader source that includes exactly one two-byte utf-8 character.