shlwapi: PathCommonPrefixA shouldn't empty achPath if lpszFile1 and/or lpszFile2 are empty.

[Christian Inci]

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Maybe, the same behaviour applies to PathCommonPrefixW, but I didn't tested it as there aren't any tests for it.
Actually, I haven't any idea what get fixed with this patch except, for sure, two testcases.
Note: I, intentionally, didn't reformatted these two lines in the testcase.

Signed-off-by: Christian Inci <chris.pcguy.inci at gmail.com>
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