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
'number' 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 either undefined (when the input
is invalid) or the parsed value (when the input is valid). Thus, when the value itself is
undefined, the NgModelController is tricked into believing that there is a native validation error.

Closes #9106