[5.1] Fixed the aggregate method

[GrahamCampbell]

It is marked as only returning integers and floats, but it also can return null and string. That's not the correct behaviour.

[ganxiangdong]

May be you should use “===” replace "!=="

[JoostK]

This should be reverted. Some aggregate functions can deal with strings and any existing application using this functionality is now broken.

Furthermore, as @ganxiangdong mentioned this PR is bugged.

@GrahamCampbell you are always very strict about adding tests and preventing behaviour changes in patch releases—which is good, don't get me wrong—yet completely fail to go by your own rules here. I don't understand.

[GrahamCampbell]

Why is the return annotation saying you cannot return anything but an int or a float then?

[GrahamCampbell]

This was an intentional behaviour change to correct it to match the documented return type.

[JoostK]

@GrahamCampbell I don't know why the annotation is like that, but why change the implementation in favour of changing the annotated return value?

[GrahamCampbell]

Ok, i'll fix this.

[GrahamCampbell]

Please see #14793.

[GrahamCampbell]

Basically reverted this.