Update _progress_bar.py

[cartland]

This library crashes when total_imaginary_width == 0.0 because the modulo cannot divide by zero. Although it's probably a bug if the a user of the library is trying to render something with width == 0, the library doesn't need to crash here.

We can use start = 0.0 as a reasonable value for a width of 0.

I came across this issue because I am using a collection of libraries that ultimately depend on this library. Since I'm not an owner of the libraries using this API, I thought it would be easiest to address this crash instead of changing the usage of libraries that depend on this one.

Please review the following checklist.

• [?] Docstrings on all new or modified functions / classes: I don't know enough about the context for render_indeterminate() or RenderResult to provide a useful docstring. I am just trying to fix a crash.
• [?] Updated documentation: I do not know enough about the documentation to determine if this needs additional documentation.
• [?] Updated CHANGELOG.md (where appropriate): I do not know enough about the prevalence of this issue to know if it's worth a CHANGELOG.md update.

[willmcgugan]

I'll need a regression test for this.