Fix ZeroDivisionError on zero operands in least_common_multiple

[CharlesCNorton]

Describe your change:

least_common_multiple_slow and least_common_multiple_fast raise ZeroDivisionError when either operand is 0:

• least_common_multiple_slow(0, 5), least_common_multiple_slow(5, 0), and least_common_multiple_slow(0, 0) divide by zero in the common_mult % first_num / common_mult % second_num loop guard.
• least_common_multiple_fast(0, 0) divides by greatest_common_divisor(0, 0) == 0.

The least common multiple is 0 whenever an operand is 0, which matches the standard library (math.lcm(0, 5) == 0, math.lcm(0, 0) == 0). Both functions now return 0 in that case before the division, so they agree with math.lcm and with each other. Behavior for non-zero inputs is unchanged. Doctests for the zero cases are added.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.