Get rid of @Nullable on the injected client id

This allows us to inject an optional once, in FlowRunner, and
inject a non-null value in the flows (not done yet, after this
goes in).

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133130485

javatests/google/registry/flows/FlowRunnerTest.java

@@ -123,7 +123,7 @@ public class FlowRunnerTest extends ShardableTestCase {
 
   @Test
   public void testRun_reportingLogStatement_noClientId() throws Exception {
-    flowRunner.clientId = null;
+    flowRunner.clientId = "";
     flowRunner.run();
     assertThat(parseJsonMap(findLogMessageByPrefix(handler, "EPP-REPORTING-LOG-SIGNATURE: ")))
         .containsExactly(