Fix for null mimetypes in os_files alter #3513

Made sure to check if a mimetype was null before array merging it.  When they are empty, use the array we would have merged with.
Merge branch 'SCHOLAR-3.x' into 3513-os_files_null_merge

Conflicts:
	openscholar/modules/os/modules/os_files/os_files.module
Merge branch 'SCHOLAR-3.x' into 3513-os_files_null_merge

Conflicts:
	openscholar/modules/os/modules/os_files/os_files.module
Merge branch 'SCHOLAR-3.x' into 3513-os_files_null_merge
Fixed typo leftover from merge #3513
Merge pull request #3539 from openscholar/3513-os_files_null_merge

3513 os files null merge