Which issue does this PR close?

N/A.

Rationale for this change

While reviewing apache/datafusion-python#1461, I noticed that an example in the map_extract function was wrong:

-- example
SELECT map_extract(MAP {'x': 10, 'y': NULL, 'z': 30}, 'y');
----
[]

-- datafusion
SELECT map_extract(MAP {'x': 10, 'y': NULL, 'z': 30}, 'y');
----
[NULL]

What changes are included in this PR?

• Fixed the previous example.
• Also added a new example showing a map_extract on an empty key.

Are these changes tested?

Yes.

Are there any user-facing changes?

No.