fix(compiler): remove attributes when expression in [attr.foo]='exp' …

[pkozlowski-opensource]

…evaluates to null

Fixes #4150

[pkozlowski-opensource]

@kegluneq this fixes a regression introduced in 6c3c606 but since there is no test for the mentioned patch I'm not sure if I'm not re-introducing the issue you were trying to fix originally. Would be awesome to have your input.

[kegluneq]

The original issue had to do with the type signature of setElementAttribute where the attribute value is declared as a string. Passing non-string to it caused exceptions.

This change lgtm.

[mary-poppins]

Merging PR #4163 on behalf of @vicb to branch presubmit-vicb-pr-4163.

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