JavaCompiler Diagnostic objects might not include a source

Today I was working with a project that uses the JavaCompiler API for in-memory compilation of Java sources. It accesses the compiler's Diagnostics to report compilation errors. It assumed that getSource would return a source file for each diagnostic, but that is not the case and it choked on a warning about a lack of annotation processor for a certain annotation. Initially I just wrote an inert annotation processor to squelch the warning, but then I went through the project that was the direct cause and fixed it to not assume a specific source for each diagnostic.