mgmt: allow functions to return None if no data is to be returned
mgmt: allow functions to return None if no data is to be returned

Don't force `return ''` everywhere.

QubesOS/qubes-issues#2622

woju/qubes-core-admin,QubesOS/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin