[CLEANUP] Use null for unset value in DeclarationBlock::parse()

[JakeQZ]

The variable is either a string or it isn't.
The best practice for when it isn't is for it to be null or unset().

Using false for when it isn't can lead to type-safety issues. Reference:
https://vulke.medium.com/when-should-variables-be-null-false-undefined-or-an-empty-string-in-php-4ebd73c7a954

Also use === in a comparison in the affected code.

Resolves #975.
Part of #976.

[coveralls]

coverage: 53.68% (-0.03%) from 53.705%
when pulling 85e70b7 on cleanup/declaration-block-parse/null
into afdd72f on main.

[JakeQZ]

It's possible to use === null and = null instead of isset() and unset(), if that's preferred. But given PHP offers these functions, why not use them?

[oliverklee]

It's possible to use === null and = null instead of isset() and unset(), if that's preferred. But given PHP offers these functions, why not use them?

I slightly prefer setting them to null.

But what I find most important is that we keep things consistent, i.e., use isset and unset if it's about set/not set variables, and use null checks for cases where a variable is set, but might be null. (This is what we're doing here.)