fix(input): fix false initial 'number' validation error in input[number]...

... with observed attributes

When no value has been set for an input[number] it's viewValue is undefined. This lead to false
parse validation errors, when `ctrl.$validate()` was fired before interacting with the element. A
typical situation of this happening is when other directives on the same element `$observe()`
attribute values. (Such directives are ngRequired, min, max etc.)
More specifically, the parser for native HTML5 validation returns undefined when the input is
invalid and returns the value unchanged when the input is valid. Thus, when the value itself is
undefined, the NgModelController is tricked into believing there is a native validation error.

Closes #9106