Don't let PDBSourceLineWriter::GetSymbolFunctionName return empty function names
Don't let PDBSourceLineWriter::GetSymbolFunctionName return empty function names

It's possible for `IDiaSymbol::get_name` to return S_OK and provide
and empty string. I haven't figured out the exact root cause yet
(the symbols in question are coming from the Rust standard library),
but FUNC lines with missing function names break the processor and
so we should never do it. This change makes it output "<name omitted>"
which matches the behavior of the DWARF dumping code.

R=mark@chromium.org
BUG=https://bugzilla.mozilla.org/show_bug.cgi?id=1272278

Review URL: https://codereview.chromium.org/1985643004 .

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