Fix: Handle empty namespace prefixes

[localheinz]

This PR

• demonstrates a bug (or feature) with json_decode on PHP versions prior to 7.1
• requires PHP 7.1

💁‍♂️ According to https://getcomposer.org/doc/04-schema.md#psr-4 it's perfectly fine to use an empty namespace prefix as a fallback:

If you want to have a fallback directory where any namespace will be looked for, you can use an empty prefix like:

{
   "autoload": {
       "psr-4": { "": "src/" }
   }
}

However, this is a problem with json_encode() on versions prior to PHP 7.1.

See

• https://bugs.php.net/bug.php?id=46600
• https://3v4l.org/AbCjB

Possible workarounds could be

• use json_decode($json, true), but has consequences when $json contains empty objects/arrays as both will be interpreted as empty arrays
• require PHP 7.1

[soullivaneuh]

If the usage of json_decode is problematic with PHP 7.0, bumping to PHP 7.1 would the better choice IMHO.

Plus, it semver compliant and you are still under unstable 0.x versionning, so go for it! 👍

[codecov]

Codecov Report

Merging #30 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master    #30   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity       50     50           
=======================================
  Files             7      7           
  Lines           172    172           
=======================================
  Hits            172    172

Impacted Files	Coverage Δ	Complexity Δ
src/Normalizer/VersionConstraintNormalizer.php	100% <100%> (ø)	9 <0> (ø)	⬇️

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[localheinz]

Thank you, @soullivaneuh!