[vm/fuzzer] Fix reproduction output

Rationale:
Add space between fp and seed parameter. Check whether generate variable
is set in TestRunnerKBC.
Change-Id: I3f1b21b65e75286d2ca560d9fa8c05ed3e4e5f55
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/116542
Reviewed-by: Aart Bik <ajcbik@google.com>
Commit-Queue: Felicitas Hetzelt <felih@google.com>

runtime/tools/dartfuzz/dartfuzz_test.dart

@@ -239,8 +239,10 @@ class TestRunnerKBC implements TestRunner {
   }
 
   void printReproductionCommand() {
-    print(
-        [generate, '--gen-bytecode', platform, '-o', dill, fileName].join(" "));
+    if (generate != null) {
+      print([generate, '--gen-bytecode', platform, '-o', dill, fileName]
+          .join(" "));
+    }
     print(cmd.join(" "));
   }
 
@@ -485,7 +487,7 @@ class DartFuzzTest {
 
   void showReproduce() {
     print("\n-- BEGIN REPRODUCE  --\n");
-    print("dartfuzz.dart --${ffi ? "" : "no-"}ffi --${fp ? "" : "no-"}fp"
+    print("dartfuzz.dart --${ffi ? "" : "no-"}ffi --${fp ? "" : "no-"}fp "
         "--seed ${seed} $fileName");
     print("\n-- RUN 1 --\n");
     runner1.printReproductionCommand();