glocalfileinfo: Ensure we always sniff some data to get the content type
In case the XDG database is not initialized yet we may try to sniff a
0-length data, making our content-type routines to mark non-empty files
as application/x-zerosize
.
This is clearly wrong, so in case the sniff size is not set, let's just try to read the default value. To avoid false-application/x-zerosize results.
Fixes: #2742 (closed)