[NFC] Minor function extraction in report class.

[eileenmcnaughton]

Overview

Small code simplification

Before

One long function

After

generateFilterClause is extracted

Technical Details

This mirrors a similar refactor in the extended reports class (which also
allows it to be called from more than one place.

Note I checked & there is no code difference between returning NULL & an empty string. I only went with the latter because it met the return string phpdoc

Comments

[civibot]

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[seamuslee001]

Adding merge on pass as this seems a straightforward refactor and also there are a number of unit tests on reports and if there is an issue here those unit tests should show a problem

[eileenmcnaughton]

Thanks @seamuslee001

[seamuslee001]

Jenkins re test this please