Don't create pyproject.toml unnecessarily when creating requirements.txt #894

Motivation
We shouldn't need pyproject.toml - if there are no dependencies declared in pyproject.toml we can just go ahead and create the requirements file assuming there are no dependencies (actually, in reality there might be dependencies declared in setup.cfg but that's a separate issue.) Alternatively, we might infer dependencies in a more advanced approach: #893.

Summary
Don't create pyproject.toml unnecessarily when creating requirements.txt.