fix(ngClass): should properly remove classes without fallback

[CaerusKaru]

In cases where a fallback ngClass is not defined, the directive had
no way of knowing to remove its ngClass on deactivation because the
removeStyles method has no effect on ngClass (this is the default
for all other directives). By specifying an empty default fallback, we
ensure that on deactivation, if no other default is specified, the
directive will correctly fallback to an empty string case.

Fixes #992

[ThomasBurleson]

In the commit body, would you explain why this fix works ?

[CaerusKaru]

@ThomasBurleson added

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