Restore ability to delete aggregate metadata

Commit I2c778f2237ba5bd2aa8335a0eae80f3aad3e9157 tightened the
input validation for setting the metadata for an
aggregate. However it also accidentally excluded sending
null as the metadata value which has previously allowed the
deletion of the metadata key rather than an update. This
changeset relaxes the input validation to allow deletion of
aggregate metadata again

Change-Id: I54b91c2d421243e7e7521f2352598f73a258b88a
Closes-Bug: 1334846
Merge "Restore ability to delete aggregate metadata"