Fix step param in dvclive.log method

[naibatsuteki]

Improve conditional expression checking step param value.

Closes #129

[daavoo]

Thanks for the contribution!

Just a minor thing, could you:

• Add a regression test (test that fails because of the bug and passes after the fix)

[codecov-commenter]

Codecov Report

Merging #130 (ec61992) into master (b24fda7) will not change coverage.
The diff coverage is 100.00%.

❗ Current head ec61992 differs from pull request most recent head 2e81366. Consider uploading reports for the commit 2e81366 to get more accurate results

@@           Coverage Diff           @@
##           master     #130   +/-   ##
=======================================
  Coverage   88.69%   88.69%           
=======================================
  Files          11       11           
  Lines         292      292           
=======================================
  Hits          259      259           
  Misses         33       33           

Impacted Files	Coverage Δ
dvclive/metrics.py	94.39% <100.00%> (ø)

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