Fixed return type on Phalcon\Mvc\View\Engine\AbstractEngine::partial

[CameronHall]

Hello!

• Type: bug fix
• Link to issue: [BUG]: Fatal error: Return value of Phalcon\Mvc\View\Engine\AbstractEngine::partial() must be of the type string, null returned in Unknown on line 0 Call Stack #14429

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I have updated the relevant CHANGELOG
• I have created a PR for the documentation about this change

Small description of the change: This issue has only come up now because return types weren't being enforced in Phalcon 3.

Thanks

[codecov]

Codecov Report

Merging #14447 into 4.0.x will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##            4.0.x   #14447   +/-   ##
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  Coverage   67.71%   67.71%           
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  Files         482      482           
  Lines      111207   111207           
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  Hits        75304    75304           
  Misses      35903    35903

[ruudboon]

If you have time it would be nice to have a test for this also.

[niden]

Thank you @CameronHall