[testrunner] Fix runFuchsiaTests() bug

Short-circuit and return nil if no fuchsia tests are present.

Change-Id: If99c1284909c6adb8b05a4c523332ee9ef464fdf
diff --git a/cmd/testrunner/main.go b/cmd/testrunner/main.go
index 97b3315..60b6b88 100644
--- a/cmd/testrunner/main.go
+++ b/cmd/testrunner/main.go
@@ -217,6 +217,10 @@
 }
 
 func runFuchsiaTests(tests []testsharder.Test, output *TestRunnerOutput, nodename, sshKey string) error {
+	if len(tests) == 0 {
+		return nil
+	}
+
 	if nodename == "" {
 		return errors.New("NODENAME must be set")
 	} else if sshKey == "" {