[SingleUpload] Handle required option if set to true

[sescandell]

See issue symfony2admingenerator/AdmingeneratorGeneratorBundle#803 (comment)

[ioleo]

I'm adding the low-priority label, as this does not break any functionality, and adding a solution would probably require some additional Javascript... just to handle the HTML5 require attribute.. which seems way overhead to me.

[sescandell]

👍