fix(datastore): Prevent panic on GetMulti failure

[bhshkh]

Issue:
When GetMulti fails, it causes below panic because response is nil and library is trying to access Transaction field of nil

=== RUN   TestIntegration_GetMulti
    integration_test.go:434: rpc error: code = Unauthenticated desc = transport: per-RPC creds failed due to error: oauth2: cannot fetch token: 400 Bad Request
        Response: {"error":"invalid_grant","error_description":"Invalid JWT Signature."}
--- FAIL: TestIntegration_GetMulti (0.09s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x1b1e5f3]

goroutine 237 [running]:
testing.tRunner.func1.2({0x1ca3ca0, 0x24e14b0})
	/usr/local/go/src/testing/testing.go:1545 +0x3f7
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1548 +0x716
panic({0x1ca3ca0?, 0x24e14b0?})
	/usr/local/go/src/runtime/panic.go:920 +0x270
cloud.google.com/go/datastore.(*Client).get(0xc0002a8690, {0x1f3bee0, 0xc000701c20}, {0xc0002aeac0, 0x6, 0x10c9369?}, {0x1c450a0?, 0xc0001b25b8}, 0x0)
	/Users/bahaaiman/Documents/CNDB-Workspace-2/google-cloud-go/datastore/datastore.go:507 +0x1133
cloud.google.com/go/datastore.(*Client).GetMulti(0xc0002a8690, {0x1f3be70, 0x2537ba0}, {0xc0002aeac0, 0x6, 0x8}, {0x1c450a0, 0xc0001b25b8})
	/Users/bahaaiman/Documents/CNDB-Workspace-2/google-cloud-go/datastore/datastore.go:431 +0x51b
cloud.google.com/go/datastore.TestIntegration_GetMulti(0xc000376ea0)
	/Users/bahaaiman/Documents/CNDB-Workspace-2/google-cloud-go/datastore/integration_test.go:437 +0x12ad
testing.tRunner(0xc000376ea0, 0x1e08278)
	/usr/local/go/src/testing/testing.go:1595 +0x262
created by testing.(*T).Run in goroutine 1
	/usr/local/go/src/testing/testing.go:1648 +0x846
exit status 2
FAIL	cloud.google.com/go/datastore	1.099s

The above failure can be reproduced by using expired service account key while running integration tests.
Fix:
response will not be nil when err is nil.
So, access field after checking err is not nil
After fix:

=== RUN   TestIntegration_GetMulti
    integration_test.go:434: rpc error: code = Unauthenticated desc = transport: per-RPC creds failed due to error: oauth2: cannot fetch token: 400 Bad Request
        Response: {"error":"invalid_grant","error_description":"Invalid JWT Signature."}
    integration_test.go:443: client.GetMulti got *status.Error, expected MultiError
--- FAIL: TestIntegration_GetMulti (0.08s)

[bhshkh]

Do not merge until release freeze ends in mid April