Don't fail on empty docstrings

[matt-hayden]

Hello,

Invoke v1.5.0 on Python 2.7. Methods defined without a docstring, or with an empty docstring, cause an exception at .splitlines()[0]. This doesn't cause a problem running the subcommands themselves, but --list fails.

Example: run fab --list with this:

import fabric

@fabric.tasks.task
def foo(ctx):
    """
    """
    print("noop")

[matt-hayden]

@bitprophet , is this worth keeping open?

[jefftriplett]

I will defer to @bitprophet, but it seems like a reasonable edge-case.

[jefftriplett]

@bitprophet would you prefer this return None?

[bitprophet]

Thanks for noticing this! I ended up making my own commit:

• IMO the "right" fix here is to try a strip() on the docstring in the early test; this lets us always return either None-or-string (as Jeff T noted). Bit less code, bit cleaner/more obvious.
• The docstring of this method (ironically) was inaccurate re: that issue (None vs empty string) so I also updated it
• Changelog entry
• Committed to release branch and merged to main branch