Skip generating an empty (broken) test case.

It's possible that all the cases in a given batch are excluded if
the offsets line up just right, which will cause the test to fail.
Don't generate an invalid test in this case.

PiperOrigin-RevId: 345276588

test/runtimes/runner/lib/lib.go

@@ -122,6 +122,10 @@ func getTests(ctx context.Context, d *dockerutil.Container, lang, image string,
 			}
 			tcs = append(tcs, tests[tc])
 		}
+		if len(tcs) == 0 {
+			// No tests to add to this batch.
+			continue
+		}
 		itests = append(itests, testing.InternalTest{
 			Name: strings.Join(tcs, ", "),
 			F: func(t *testing.T) {