null or empty.

Author: rmehta19

oauth2_http/java/com/google/auth/oauth2/ComputeEngineCredentials.java

@@ -50,6 +50,7 @@
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Joiner;
 import com.google.common.base.MoreObjects.ToStringHelper;
+import com.google.common.base.Strings;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
@@ -208,10 +209,10 @@ String createTokenUrlWithScopes() {
     if (!scopes.isEmpty()) {
       tokenUrl.set("scopes", Joiner.on(',').join(scopes));
     }
-    if (!transport.isEmpty()) {
+    if (!Strings.isNullOrEmpty(transport)) {
       tokenUrl.set("transport", transport);
     }
-    if (!bindingEnforcement.isEmpty()) {
+    if (!Strings.isNullOrEmpty(bindingEnforcement)) {
       tokenUrl.set("binding-enforcement", bindingEnforcement);
     }
     return tokenUrl.toString();

oauth2_http/javatests/com/google/auth/oauth2/ComputeEngineCredentialsTest.java

@@ -190,6 +190,54 @@ public void buildTokenUrlWithScopes_defaultScopes() {
     assertEquals("bar", scopes.toArray()[1]);
   }
 
+  @Test
+  public void buildTokenUrl_emptyTransport() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setTransport("")
+            .setBindingEnforcement("abc")
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL +  "?binding-enforcement=abc", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrl_nullTransport() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setTransport(null)
+            .setBindingEnforcement("abc")
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL +  "?binding-enforcement=abc", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrl_emptyBindingEnforcement() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setTransport("abc")
+            .setBindingEnforcement("")
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL +  "?transport=abc", softBoundTokenUrl);
+  }
+
+  @Test
+  public void buildTokenUrl_nullBindingEnforcement() {
+    ComputeEngineCredentials credentials =
+        ComputeEngineCredentials.newBuilder()
+            .setTransport("abc")
+            .setBindingEnforcement(null)
+            .build();
+    String softBoundTokenUrl = credentials.createTokenUrlWithScopes();
+
+    assertEquals(TOKEN_URL +  "?transport=abc", softBoundTokenUrl);
+  }
+
   @Test
   public void buildTokenUrlSoftMtlsBound_mtls_transport() {
     ComputeEngineCredentials credentials =