Make PFMERGE source key optional in docs, add tests with one input key, add tests on missing source keys (#11205)

The following two cases will create an empty destkey HLL:
1. called with no source keys, like `pfmerge destkey`
2. called with non-existing source keys, like `pfmerge destkey non-existing-source-key`

In the first case, in `PFMERGE`, the dest key is actually one of the source keys too.
So `PFMERGE k1 k2` is equivalent to `SUNIONSTORE k1 k1 k2`,
and `PFMERGE k1` is equivalent to `SUNIONSTORE k1 k1`.
So the first case is reasonable, the source key is actually optional.

And the second case, `PFMERGE` on missing keys should succeed and create an empty dest.
This is consistent with `PFCOUNT`, and also with `SUNIONSTORE`, no need to change.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/commands/pfmerge.json b/src/commands/pfmerge.json
index ae18dad1e..648a8b43a 100644
--- a/src/commands/pfmerge.json
+++ b/src/commands/pfmerge.json
@@ -62,6 +62,7 @@
                 "name": "sourcekey",
                 "type": "key",
                 "key_spec_index": 1,
+                "optional": true,
                 "multiple": true
             }
         ]