Add lemmas `merge_empty_l` and `merge_empty_r`.

2 jobs for !280 with robbert/misc_map in 5 minutes and 13 seconds (queued for 4 seconds)
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passed #131500
fp
build-coq.8.10.2

00:05:13

passed #131499
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build-coq.8.13.2-mr

00:04:44