-ddump-to-file should create empty dump files when there was nothing to dump

For example, I ran with -ddump-rule-firings -ddump-to-file when there were no rule firings. GHC should have created an empty foo.dump-rule-firings file. (Instead it left the existing foo.dump-rule-firings from a previous run intact, which could have been misleading if I didn't already know that the most recent run had no rule firings.)