Replace `len()` Zero Checks with Pythonic Empty Sequence Checks

[andylizf]

Replace len() comparisons with 0/1 with pythonic empty sequence checks, following Google Python Style Guide's recommendation:

For sequences (strings, lists, tuples), use the fact that empty sequences are false, so if seq: and if not seq: are preferable to if len(seq): and if not len(seq): respectively.

Also fixed some typing annotations along the way.

Tested (run the relevant ones):

• [x] Code formatting: bash format.sh
• [ ] Any manual or new tests for this PR (please specify below)
• [ ] All smoke tests: pytest tests/test_smoke.py
• [ ] Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• [ ] Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[andylizf]

@cblmemo PTAL, thanks!

[Michaelvll]

Please refer to the style guide: https://google.github.io/styleguide/pyguide.html#214-truefalse-evaluations

[andylizf]

@Michaelvll Precisely. As mentioned, this modification was just made based on the Google Python Style Guide. Please refer to: https://google.github.io/styleguide/pyguide.html#2144-decision.

[andylizf]

@Michaelvll By the way, when you have a moment, could you take a look at #4250 ? Thanks a lot!

[Michaelvll]

Ahh, thanks for fixing these! I misread the message. Looks pretty good at the first glance. cc'ing @cblmemo to take a closer look. : )