Fix crash when creating node pools with name_prefix or no name

[danawillow]

Fixes #479.

$ make testacc TEST=./google TESTARGS='-run=TestAccContainerNodePool_noName'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./google -v -run=TestAccContainerNodePool_noName -timeout 120m
=== RUN   TestAccContainerNodePool_noName
--- PASS: TestAccContainerNodePool_noName (573.96s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	574.179s

$ make testacc TEST=./google TESTARGS='-run=TestAccContainerNodePool_namePrefix'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./google -v -run=TestAccContainerNodePool_namePrefix -timeout 120m
=== RUN   TestAccContainerNodePool_namePrefix
--- PASS: TestAccContainerNodePool_namePrefix (614.92s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	615.102s

$ make testacc TEST=./google TESTARGS='-run=TestAccContainerCluster_withNodePoolNamePrefix'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./google -v -run=TestAccContainerCluster_withNodePoolNamePrefix -timeout 120m
=== RUN   TestAccContainerCluster_withNodePoolNamePrefix
--- PASS: TestAccContainerCluster_withNodePoolNamePrefix (387.70s)
PASS
ok  	github.com/terraform-providers/terraform-provider-google/google	387.860s

[ghost]

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