fix(input): false is no longer an empty value by default

[petebacondarwin]

This builds upon and improves @lgalfaso's PR #3658, without resorting to hard-coding fixes across directives.

checkboxInputType and ngList directives need to have special logic for whether
they are empty or not. Previously this had been hard coded into their
own directives or the ngRequired directive. This made it difficult to handle
these special cases.

This change factors out the question of whether an input is empty into a method
$isEmpty on the ngModelController. The ngRequired directive now uses this
method when testing for validity and directives, such as checkbox or ngList
can override it to apply logic specific to their needs.

Closes #3490, #3658, #2594

[mary-poppins]

Thanks for the PR!

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[lgalfaso]

Nice patch!

[pkozlowski-opensource]

+1, very nice

[IgorMinar]

Wouldn't it be possible to provide an API for this that doesn't require monkey-patching? This functionality is needed, but I'm not keen on the API you created.

[petebacondarwin]

@IgorMinar - can you give some more detail? There is no monkey patching going on here. Instead, I am creating an interface that allows input directives to specify what it means to them to be "empty".

This is done through the polymorphic method $isEmpty() on ngModelController, in just the same way that directives can provide their own version of $render().

[petebacondarwin]

OK, so I guess, strictly, it is monkey patching individual instances of ngModelController but this is exactly what $render() does too.

[vojtajina]

@petebacondarwin the isEmpty does not have to be exposed on the controller. How about if we only keep the local isEmpty and the directives that need a different implementation will use their own.

[vojtajina]

Ignore my previous comment, I just realized that ng-required needs to read it.

[vojtajina]

@petebacondarwin how about this vojtajina@c44fcb6 (it's one commit on the top of your PR)

[vojtajina]

Made some little tweaks and merged as b56b21a.

[petebacondarwin]

@vojtajina - Great! Thanks.