fix(ngClass): do not break on invalid values

Previously, when an `ngClass` expression evaluated to something that was
not a string, array or object (and was truthy), an error would be thrown
while trying to call `.split()` on a non-string value. This error was
not very helpful for the user to identify the root cause of the problem.

This commit fixes it by ensuring such values are converted to string.

Fixes #16697

Closes #16699