Handle TypeError exception when no ImageMagick install is present
Handle TypeError exception when no ImageMagick install is present

Fixes an error introduced in 1a6e0a7 where a TypeError exception was
raised when calling `_check_method()` with no ImageMagick installation
present.

jackwilsdon/beets,sampsyo/beets,shamangeorge/beets,beetbox/beets,ibmibmibm/beets,SusannaMaria/beets,SusannaMaria/beets,sampsyo/beets,beetbox/beets,jackwilsdon/beets,sampsyo/beets,sampsyo/beets,ibmibmibm/beets,SusannaMaria/beets,beetbox/beets,beetbox/beets,jackwilsdon/beets,ibmibmibm/beets,ibmibmibm/beets,jackwilsdon/beets,SusannaMaria/beets,shamangeorge/beets,shamangeorge/beets,shamangeorge/beets