test: add nil checks

[bhshkh]

Without these nil checks, the test panics and further tests are not run. E.g.:

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=0x28 pc=0x150ed49]

goroutine 93 [running]:
testing.tRunner.func1.2({0x15a7020, 0x1ad7d40})
	/usr/local/go/src/testing/testing.go:1526 +0x24e
testing.tRunner.func1()
	/usr/local/go/src/testing/testing.go:1529 +0x39f
panic({0x15a7020, 0x1ad7d40})
	/usr/local/go/src/runtime/panic.go:884 +0x213
github.com/googleapis/cloud-bigtable-clients-test/tests.TestCheckAndMutateRow_Generic_CloseClient(0xc00041c000)
	*****************************/cloud-bigtable-clients-test/tests/checkandmutaterow_test.go:299 +0x669
testing.tRunner(0xc00041c000, 0x16990d0)
	/usr/local/go/src/testing/testing.go:1576 +0x10b
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1629 +0x3ea
exit status 2

[liujiongxin]

There are other *_test.go files that may need this too. You may apply it to all of them for consistency?