Fix user-email validation

[kevinpapst]

Description

Fixes #2003

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I verified that my code applies to the guidelines (composer code-check)
• I updated the documentation (see here)
• I agree that this code is used in Kimai and will be published under the MIT license

[codecov]

Codecov Report

Merging #2004 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #2004      +/-   ##
============================================
- Coverage     93.38%   93.37%   -0.01%     
  Complexity     5461     5461              
============================================
  Files           516      516              
  Lines         16762    16762              
============================================
- Hits          15653    15652       -1     
- Misses         1109     1110       +1     

Impacted Files	Coverage Δ	Complexity Δ
src/Form/UserCreateType.php	100.00% <ø> (ø)	3.00 <0.00> (ø)
src/Repository/TimesheetRepository.php	89.78% <0.00%> (-0.20%)	128.00% <0.00%> (ø%)