docs(input): Removed invalid statement

[ProLoser]

$parsers should return values regardless of validity to allow chaining validation.

If a $parser returns undefined then sequential parsers will all automatically fail and the validation message feedback will be inaccurate.

See this discussion: angular-ui/ui-utils#149

[mary-poppins]

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[caitp]

The intended behaviour is that the parsed value becomes undefined, and therefore the model value is undefined, indicating an invalid value. This should not harm other validators in the chain, because they should ignore values deemed empty by ngModelController.$isEmpty(value), which should account for the undefined case. (The exception to this would be the ngRequired validator, but its parser is unshifted onto the parser array and so should be evaluated early)

[tbosch]

Well, you can't parse invalid values, e.g. parse a non numeric value as a numeric value would just not work. So I think this should stay here...