Fix bug with external at-rule in conditional.

If an @external rule and the definition of the style class associated
are contained in a conditional node evaluated to false at compile
time and the style class are not associated to a java method (valid
for an external style class), the compiler throws an error "The
following non-obfuscated class is present in a strict CssResource"

Change-Id: Ia110a0dfedf72babc72ce2a2f9126e23bd2a94dc