On linux SndFileSource::get_soundfile_info was being called with an empty path
value when clicking on a folder in the left side part of the file browser which
resulted in an unnecessary error message.
On Windows every time a directory was selected in the Import dialog,
SndFileSource::get_soundfile_info would fail to open the directory in read only
mode and produce an error, but as we don't want to query/open soundfile info
for directories anyway just test and return if the path is a directory.
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