fix: Check mutation error only if exists

[bhshkh]

TestMutateRows_Generic_DeadlineExceeded fails for Go client library with below error

=== RUN   TestMutateRows_Generic_DeadlineExceeded
[Servr log] 2025/01/15 19:19:42 There is 10s sleep on the server side
2025/01/15 19:19:44 received error from Table.ApplyBulk(): rpc error: code = DeadlineExceeded desc = context deadline exceeded
2025/01/15 19:19:44 error: rpc error: code = DeadlineExceeded desc = context deadline exceeded
    mutaterows_test.go:227: 
        	Error Trace:	/home/runner/work/google-cloud-go/google-cloud-go/cloud-bigtable-clients-test/tests/mutaterows_test.go:227
        	Error:      	Not equal: 
        	            	expected: 1
        	            	actual  : 0
        	Test:       	TestMutateRows_Generic_DeadlineExceeded
--- FAIL: TestMutateRows_Generic_DeadlineExceeded (2.00s)

This test expects overall operation to return error and also per mutation error but in Go, method is documented as below:

// If the entire process
// fails, (nil, err) will be returned. If specific mutations
// fail to apply, ([]err, nil) will be returned, and the errors
// will correspond to the relevant rowKeys/muts arguments.

So, I can't return ([]err, err) from Go library ApplyBulk method. This may break user's code.

Thus, updating the test itself

[andre-sampaio]

I'm rethinking what I suggested about changing the test: now that I think about it this could make other clients more brittle.

I think it would be safer to change the test-proxy implementation for the go client.

Also, I have created an issue for us to review ApplyBulk at some point: googleapis/google-cloud-go#11473

[bhshkh]

Created googleapis/google-cloud-go#11478 to update test proxy