fix: do not throw NPE from BigQueryImpl.testIamPermissions (#1596)

Fix for issue where `BigQueryImpl.testIamPermissions` will throw a NullPointerException if the caller does not have any of the permissions being checked.

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Fixes #1595 ☕️

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/BigQueryImpl.java

@@ -1495,7 +1495,9 @@ public com.google.api.services.bigquery.model.TestIamPermissionsResponse call()
               getOptions().getRetrySettings(),
               EXCEPTION_HANDLER,
               getOptions().getClock());
-      return ImmutableList.copyOf(response.getPermissions());
+      return response.getPermissions() == null
+          ? ImmutableList.of()
+          : ImmutableList.copyOf(response.getPermissions());
     } catch (RetryHelperException e) {
       throw BigQueryException.translateAndThrow(e);
     }

google-cloud-bigquery/src/test/java/com/google/cloud/bigquery/BigQueryImplTest.java

@@ -2766,4 +2766,21 @@ public void testTestIamPermissions() {
     assertEquals(perms, grantedPermissions);
     verify(bigqueryRpcMock).testIamPermissions(resourceId, checkedPermissions, EMPTY_RPC_OPTIONS);
   }
+
+  @Test
+  public void testTestIamPermissionsWhenNoPermissionsGranted() {
+    final String resourceId =
+        String.format("projects/%s/datasets/%s/tables/%s", PROJECT, DATASET, TABLE);
+    final List<String> checkedPermissions = ImmutableList.<String>of("foo", "bar", "baz");
+    // If caller has no permissions, TestIamPermissionsResponse.permissions will be null
+    final com.google.api.services.bigquery.model.TestIamPermissionsResponse response =
+        new com.google.api.services.bigquery.model.TestIamPermissionsResponse()
+            .setPermissions(null);
+    when(bigqueryRpcMock.testIamPermissions(resourceId, checkedPermissions, EMPTY_RPC_OPTIONS))
+        .thenReturn(response);
+    bigquery = options.getService();
+    List<String> perms = bigquery.testIamPermissions(TABLE_ID, checkedPermissions);
+    assertEquals(perms, ImmutableList.of());
+    verify(bigqueryRpcMock).testIamPermissions(resourceId, checkedPermissions, EMPTY_RPC_OPTIONS);
+  }
 }