metric: Fix precision format

[e-nikolov]

Currently if I try to print a 2 digit number with precision 0, I get this error:
ValueError: Format specifier missing precision because the format results in a negative precision number.

This PR makes sure that the precision is at least 0

[hugovk]

Please can you share a snippet of code that reproduces the problem, and also include tests?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5cb0323) 99.19% compared to head (e5a3976) 99.19%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #159   +/-   ##
=======================================
  Coverage   99.19%   99.19%           
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  Files           9        9           
  Lines         746      746           
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  Hits          740      740           
  Misses          6        6           

Flag	Coverage Δ
macos-latest	97.58% <100.00%> (ø)
ubuntu-latest	97.58% <100.00%> (ø)
windows-latest	96.11% <100.00%> (ø)

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[e-nikolov]

@hugovk

I added a couple of tests.

A snippet that shows the problem would be something like this:

import humanize

print(humanize.metric(10, "", 0))

# expected:     10
# actual:       ValueError: Format specifier missing precision

[hugovk]

Thank you!