Deny pass null to CreateService::getTemplate()

[KalimeroMK]

Fix #313

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	✔️
Fixed issues	#313

null is never passed to CreateService::getTemplate(), so the nullable type and the (string) cast are removed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.39%. Comparing base (8e1dcdb) to head (083ea14).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@              Coverage Diff              @@
##             master     #355       +/-   ##
=============================================
+ Coverage      0.00%   96.39%   +96.39%     
  Complexity      311      311               
=============================================
  Files            20       20               
  Lines          1000      999        -1     
=============================================
+ Hits              0      963      +963     
+ Misses         1000       36      -964     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.