ApiVisualEditorEdit: Fix handling of 'minor' parameter
The client-side code sends the empty string as the value of this
parameter when the edit should be marked as minor, and doesn't set it
at all when it shouldn't (this is the normal convention for boolean
parameters). However, the API code here was incorrectly handling the
empty string, and not marking the edit as minor as a result.
This was revealed by 50883dd7fea3f06b10b2d41eb99671c885760ca5. Prior
to that change the original 'minor' parameter was forwarded to the
action=edit call if it was provided, overriding the 'notminor'
parameter, which was effectively added unconditionally.