Twenty Twelve, Twenty Fourteen: Simplify header image check

[obenland]

get_header_image() will return the boolean false or an empty string if there is no header image. Both will be evaluated as false by the if statement. We already do this in Twenty Ten.

Loosely related: #25156.

[lancewillett]

In 25604:

Twenty Fourteen: simplify header image check, props obenland. See #25400.

[lancewillett]

In 25626:

Twenty Twelve: simplify header image check, props obenland. Closes #25400.