Fix TranslationUnit.FileRelativePath for invalid units.
Fix TranslationUnit.FileRelativePath for invalid units.

zillemarco/CppSharp,ddobrev/CppSharp,ddobrev/CppSharp,mono/CppSharp,mono/CppSharp,ddobrev/CppSharp,zillemarco/CppSharp,ddobrev/CppSharp,mono/CppSharp,ddobrev/CppSharp,zillemarco/CppSharp,mono/CppSharp,mono/CppSharp,zillemarco/CppSharp,mono/CppSharp,zillemarco/CppSharp