Fix MemMapFs RealPath error when BasePath = ""

[phyrwork]

bpath cleans to ".", but paths "./" clean to "".

This results in "." not found as prefix of "*" except for hidden files and dirs.

Skip the prefix check if bpath == "." as this is lexically equivalent to "", which would satisfy the check.

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[0xmichalis]

A unit test case that covers the change would be nice.