Fix a backward compatibility bug.

To remain backward compatible, we only set port if it's a non-emptry
string.  But, we always store host.

src/core/lib/gprpp/host_port.cc

@@ -93,8 +93,12 @@ bool SplitHostPort(StringView name, UniquePtr<char>* host,
   StringView port_view;
   const bool ret = SplitHostPort(name, &host_view, &port_view);
   if (ret) {
+    // We always set the host, but port is set only when it's non-empty,
+    // to remain backward compatible with the old split_host_port API.
     *host = host_view.dup();
-    *port = port_view.dup();
+    if (!port_view.empty()) {
+      *port = port_view.dup();
+    }
   }
   return ret;
 }

test/core/gprpp/host_port_test.cc

@@ -68,10 +68,10 @@ static void split_host_port_expect(const char* name, const char* host,
 }
 
 static void test_split_host_port() {
-  split_host_port_expect("", "", "", true);
-  split_host_port_expect("[a:b]", "a:b", "", true);
-  split_host_port_expect("1.2.3.4", "1.2.3.4", "", true);
-  split_host_port_expect("a:b:c::", "a:b:c::", "", true);
+  split_host_port_expect("", "", nullptr, true);
+  split_host_port_expect("[a:b]", "a:b", nullptr, true);
+  split_host_port_expect("1.2.3.4", "1.2.3.4", nullptr, true);
+  split_host_port_expect("a:b:c::", "a:b:c::", nullptr, true);
   split_host_port_expect("[a:b]:30", "a:b", "30", true);
   split_host_port_expect("1.2.3.4:30", "1.2.3.4", "30", true);
   split_host_port_expect(":30", "", "30", true);