Update objective_function_input_checks.R to remove null dependent-arg-function bug

[leavesandlemmas]

This PR fixes #14. It's a same change that adds another check is.null, although there might be a way to only check once if dependent_arg_function is null. You can merge if/when satisfied.

[eloch216]

I'm going to close this since I have already made another version on a different branch I have been working on today. A few notes about this one:

• BioCroValidation uses the same git branching flow as BioCro, so this should be merged into develop rather than main.
• When dependent_arg_function is NULL, I think it's still helpful to indicate that there are no dependent arguments when operating in verbose mode, rather than printing nothing at all.
• I am trying to be good about regression tests for this package, since it is just getting started, and tests are easier to add as issues come up. Ideally, every change should have an associated test -- here it could check that no error occurs in verbose mode when no dependent argument function has been supplied. The output can be rerouted to a temporary file using sink to ensure the test does not print messages to the R terminal.

[eloch216]

See PR #16