`FUNC` with implicit `MAP` returns allowed.

[python-processing-unit]

According to the specification section 5.3:

Functions declared with return type TNS, MAP, FUNC, or THR MUST raise a runtime error if control reaches the end of the body without an explicit RETURN.

In the implementation, however, FUNC with implicit MAP returns return <null>.

Prefix REPL. Enter statements, blank line to run buffer.
>>> FUNC MAP: RETMAP(){}
>>> PRINT(RETMAP())
<null>