Handle edge case in validate script

[jrolfs]

What:

Fix a bug in the validate script.

Why:

The validate script currently assumes at least one script will be run, however I ran into an edge case where no scripts should be run which results in an error as concurrently is spawned without any script arguments.

The particular edge case in which I came across this issue:

• The validate script is being spawned from the pre-commit script
• The package in which the scripts are run has no build or flow script defined in its package.json

How:

Only spawn concurrently if more than 0 scripts are queued to run.

Checklist:

• Documentation N/A
• Tests
• Ready to be merged

[codecov]

Codecov Report

Merging #128 into master will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
+ Coverage   87.38%   87.50%   +0.11%     
==========================================
  Files          18       18              
  Lines         325      328       +3     
  Branches       75       76       +1     
==========================================
+ Hits          284      287       +3     
  Misses         34       34              
  Partials        7        7              

Impacted Files	Coverage Δ
src/scripts/validate.js	100.00% <100.00%> (ø)

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[kentcdodds]

Sounds good. Thanks 👍

[kentcdodds]

🎉 This PR is included in version 5.6.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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