[automerger skipped] strings.mk: Allow empty RHS values for inputs of collapse-pairs am: cc39735e55 am: c461e817e4 am: 5f8fd89cf1 am: aec22adf04 -s ours am: 4b31445bda -s ours

am skip reason: Change-Id I35faeaedc3bc42e56e01201baf7ea6805a610439 with SHA-1 cc39735e55 is in history

Change-Id: Id7782d3532273ea3a94bba317e38c6aa3bf75b4e