AssertContains - handle empty string in string.

[SpacePossum]

closes #2520

This PR make the following assert pass:

$this->assertContains('', 'test');

previous:

There was 1 error:

1) myTest::testA
mb_strpos(): Empty delimiter

/home/possum/work/phpunit/src/Framework/Constraint/StringContains.php:59

[localheinz]

👍

[codecov-io]

Codecov Report

Merging #2778 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #2778      +/-   ##
============================================
+ Coverage     54.84%   54.85%   +0.01%     
- Complexity     2794     2795       +1     
============================================
  Files           104      104              
  Lines          7766     7768       +2     
============================================
+ Hits           4259     4261       +2     
  Misses         3507     3507

Impacted Files	Coverage Δ	Complexity Δ
src/Framework/Constraint/StringContains.php	100% <100%> (ø)	6 <0> (+1)	⬆️

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[sebastianbergmann]

Merged manually, thanks.