Python frontend fixes for return values and arguments

[tbennun]

• Properly parse return statements without a return value
• Fix missing argument detection when no arguments are given

[tbennun]

@phschaad when reviewing, keep the following code in mind (I couldn't reproduce such a situation, you are more qualified to do so): is there any case where the top-level CFG target is an SDFG, and we still create a terminator (ReturnBlock) state?
I am thinking of a situation where the python frontend may parse an SDFG, visit a Return statement, and then continue to visit other states/blocks. In that case, the "return" state might still be incorrectly connected to subsequent states.

I want to make sure that creating a terminator (like the return statement) is airtight. Thanks!

[tbennun]

@phschaad Comment in PR refers to this code.

[phschaad]

@tbennun There isn't currently something in the frontend that consciously does it that way, but having a return block followed by other content is still valid and works similar to how it would work in Python - i.e., the program exits and everything following is dead code. On a separate branch I currently have even augmented dead state elimination to take that into account and kill more unreachable states. I have thus augmented the PR slightly to insert such a return block now to make the feature airtight.