* test empty authorized masters' cidr_blocks
When the `cidr_block` isn't simply blank but contains an empty list as in
```
master_authorized_networks_config {
cidr_blocks = []
}
```
a panic occurs looking something like
```
goroutine 26 [running]:
github.com/terraform-providers/terraform-provider-google/google.expandMasterAuthorizedNetworksConfig(0x15a4f80, 0xc4202586e0, 0x21)
/tmp/GOPATH/src/github.com/terraform-providers/terraform-provider-google/google/resource_container_cluster.go:1355 +0x4f2
github.com/terraform-providers/terraform-provider-google/google.resourceContainerClusterCreate(0xc420146a80, 0x16b1800, 0xc4200b8000, 0x0, 0x0)
/tmp/GOPATH/src/github.com/terraform-providers/terraform-provider-google/google/resource_container_cluster.go:520 +0x2848
github.com/terraform-providers/terraform-provider-google/vendor/github.com/hashicorp/terraform/helper/schema.(*Resource).Apply(0xc420495490, 0xc420341310, 0xc4202582c0, 0x16b1800, 0xc4200b8000, 0x1, 0xc42024eae0, 0xc4201e3650)
/tmp/GOPATH/src/github.com/terraform-providers/terraform-provider-google/vendor/github.com/hashicorp/terraform/helper/schema/resource.go:227 +0x364
github.com/terraform-providers/terraform-provider-google/vendor/github.com/hashicorp/terraform/helper/schema.(*Provider).Apply(0xc4204c6700, 0xc4203412c0, 0xc420341310, 0xc4202582c0, 0x14ee1441a000, 0x0, 0x18)
/tmp/GOPATH/src/github.com/terraform-providers/terraform-provider-google/vendor/github.com/hashicorp/terraform/helper/schema/provider.go:283 +0xa4
github.com/terraform-providers/terraform-provider-google/vendor/github.com/hashicorp/terraform/plugin.(*ResourceProviderServer).Apply(0xc4202d7c40, 0xc42035de80, 0xc42025c160, 0x0, 0x0)
/tmp/GOPATH/src/github.com/terraform-providers/terraform-provider-google/vendor/github.com/hashicorp/terraform/plugin/resource_provider.go:527 +0x57
reflect.Value.call(0xc4203feae0, 0xc42000e038, 0x13, 0x19e88a8, 0x4, 0xc42015ff20, 0x3, 0x3, 0xc420047ee8, 0xc4204c6798, ...)
/usr/local/go/src/reflect/value.go:434 +0x905
reflect.Value.Call(0xc4203feae0, 0xc42000e038, 0x13, 0xc420047f20, 0x3, 0x3, 0xc400000001, 0x0, 0x0)
/usr/local/go/src/reflect/value.go:302 +0xa4
net/rpc.(*service).call(0xc420418600, 0xc42007c140, 0xc42001e798, 0xc4200c4000, 0xc4202d6c40, 0x1557f80, 0xc42035de80, 0x16, 0x1557fc0, 0xc42025c160, ...)
/usr/local/go/src/net/rpc/server.go:381 +0x142
created by net/rpc.(*Server).ServeCodec
/usr/local/go/src/net/rpc/server.go:475 +0x36b
```
which we trigger by altering the first step to contain the HCL notation
for an empty list instead of simply an empty string.
In order to accomplish this, the tests had to be modified to accept an
emptyValue string as well which contains the content of the
`emptyValue` string when the cidrBlocks array is empty. This maintains
the old behavior of the original tests when `emptyValue` is an empty
string, while also facilating differing behavior for the new testcase by
setting `emptyValue` to whichever string we want to test instead. I
don't think this is very clean, but I guess it's pragmatic enough.
I'll hear if this is a thorn in the side to someone 😏.
* avoid panic on cidr_block type assertion
This is basically the fix. Since the value can be nil, we want to ensure
we handle a failure during the assertion since we know that asserting
`nil` conforms to `map[string]interface{}` will cause a run-time panic.
* flatten to config on empty list for cidr_blocks
since an empty list for cidrBlocks constitutes valid input, one should
return a map containing an empty list for the cidr_blocks field instead
of a nil value.
The nil value is only appropriate when the input Config is also nil.
* Update google_container_cluster master_auth username and password without recreation of cluster, using setMasterAuth method
* Add tests to update and disable master_auth password and username values
