daterange - allow optionnal start date

[ghost]

While testing dateRange filtering for collection, I encountered error when the start value is not specified (see #3345 for the issue).

After looking deeper in the source code, I found that the method dateRange() come from an interface and has three implementations and the fix I submited in the issue was not complete. That's why I submit this pull request.

I've divided the fix into three commit so you may discard some of them :

• First commit deal only with the fix of behaviour and touch only minimum lines of codes,
• Second commit simplify a little bit the code,
• Third commit is about the interface (method definition) : using null instead of false (as the behaviour is not specified if you pass true`), updating the documentation of the code (and synchronize the documentation accross all implemtnations) and fixing the only part of the code that call the method (with null instead of a mix of 0, false and null).

As 0, null and false have the same behaviour, the new version is retro-compatible with hypothetical legacy plugins that use the method.

The code have been manually tested on page collection with a fresh install of the fork and default theme :

• Three pages, one in the pase (1800's), one in the present (2000's) and one in the future (2200's),
• Test with start only, end only, both set,
• Same test but with the field attribute specified.

[mahagr]

FYI: it's good practice to set return value as tight as possible from the docblock -- it allows chaining the method calls without static analyzer reporting errors on undefined methods.

I wish we were using PHP 7.4 or 8.0 minimum as it would allow us to define the return values.

[ghost]

Thank you for the information about the type in "@return". I didn't know why it was different from one method to the other, now, I know :-)