Coveralls: assert that `github_pr` is either a string or an integer, and don't rely on integers being "non-empty"

[codecov[bot]]

Codecov Report

Merging #317 (239bdb6) into master (aa01a30) will increase coverage by 0.55%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
+ Coverage   90.25%   90.81%   +0.55%     
==========================================
  Files           2        2              
  Lines         195      196       +1     
==========================================
+ Hits          176      178       +2     
+ Misses         19       18       -1     

Impacted Files	Coverage Δ
src/coveralls.jl	94.23% <100.00%> (-1.89%)	:arrow_down:
src/codecovio.jl	86.95% <0.00%> (+3.26%)	:arrow_up:

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[coveralls]

Pull Request Test Coverage Report for Build 926825446

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.6%) to 90.816%

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Files with Coverage Reduction	New Missed Lines	%
src/codecovio.jl	2	86.96%
src/coveralls.jl	2	94.23%
<!--	Total:	4		-->

Totals
Change from base Build 920524320:	0.6%
Covered Lines:	178
Relevant Lines:	196

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💛 - Coveralls