improve customer validation

[kevinpapst]

Description

empty currency lead to invalid API response

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 #2083 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #2083   +/-   ##
=========================================
  Coverage     93.28%   93.28%           
- Complexity     5804     5807    +3     
=========================================
  Files           531      531           
  Lines         17571    17571           
=========================================
  Hits          16391    16391           
  Misses         1180     1180           

Impacted Files	Coverage Δ	Complexity Δ
src/Entity/Customer.php	100.00% <100.00%> (ø)	83.00 <6.00> (+3.00)