Don't show the dialog when no paste content

The dialog that is displayed upon pasting is unnecessary when no
paste content is present. To prevent it from appearing and looking
like a bug, make it not appear when pasteContentString is empty
Patchset 2: Rebase
Patchset 3: Commit message edit
Patchset 4: Should be working now
Patchset 6: Unstage a file accidentally included in commit

Change-Id: I78e59625695c8c2a2b85cea2b80fc177829b66d4