Make async flow logic handle missing client transaction IDs

Per EPP RFC 5730, the <clTRID> element is optional. However, we weren't handling
it not being specified in asynchronous contact/host deletions because we were
adding it directly as a parameter value on a task, which does not allow null and
thus threw a NullPointerException.

This fixes handling for nulls (the parameter isn't set at all) and adds a test.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=194123259

javatests/google/registry/flows/host/testdata/host_delete_no_cltrid.xml

@@ -0,0 +1,10 @@
+<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
+  <command>
+    <delete>
+      <host:delete
+       xmlns:host="urn:ietf:params:xml:ns:host-1.0">
+        <host:name>%HOSTNAME%</host:name>
+      </host:delete>
+    </delete>
+  </command>
+</epp>

javatests/google/registry/flows/host/testdata/host_delete_response_no_cltrid.xml

@@ -0,0 +1,10 @@
+<epp xmlns="urn:ietf:params:xml:ns:epp-1.0">
+  <response>
+    <result code="1001">
+      <msg>Command completed successfully; action pending</msg>
+    </result>
+    <trID>
+      <svTRID>server-trid</svTRID>
+    </trID>
+  </response>
+</epp>