find_file and find_folder do not return null string if the file or folder resp. do not exist (coding typo)

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21267 4ae1f11a-8b86-4847-b8af-ab372f36d1fd
export file select dialogue does not correctly reject empty filenames - two (probably long standing) coding errors: use of method name 'strip' instead of calling it, and does not isolate the filename from the full pathname

git-svn-id: svn+ssh://svn.code.sf.net/p/gramps/code/trunk@21268 4ae1f11a-8b86-4847-b8af-ab372f36d1fd