Fix null pointer access when iterating over empty arrays

[s-ludwig]

The PropTable.table field is a RandAA, which is a class and thus does not exhibit the more forgiving interface as built-in AAs.

This PR also fixes the CI stage for running the test262 test suite, which appears to have been broken for a while.

[WalterBright]

thank you!

[WalterBright]

What about the failing test?

[s-ludwig]

Looks like the python2 executable isn't available by default for GitHub actions anymore. I'll see if I can quickly fix that.

[s-ludwig]

Finally got it to complete again. There were several issues:

• Python 2 not being available, I first tried to install Python 2 manually, which didn't quite work, so instead the code is now adjusted for Python 3
• Some test log output caused grep to recognize the log as binary and in turn to not output the matched text
• For some reason the dmdscript:ds binary, which used by the test suite, crashes when being generated by DMD - I worked around that for now by compiling with LDC
• The fix makes some additional test cases pass, so the number needed to be bumped up

[s-ludwig]

Fixed another small issue in RandAA that came up.