tests: adjust file part behavior in test verify section.

When a <file> part is now specified with no contents at all, this
will actually verify that the specified file has no contents at all.
Previously file contents would be ignored.

tests/runtests.pl

@@ -3499,7 +3499,7 @@ sub singletest {
     my $outputok;
     for my $partsuffix (('', '1', '2', '3', '4')) {
         my @outfile=getpart("verify", "file".$partsuffix);
-        if(@outfile) {
+        if(@outfile || partexists("verify", "file".$partsuffix) ) {
             # we're supposed to verify a dynamically generated file!
             my %hash = getpartattr("verify", "file".$partsuffix);