…w key

Against a real version of Bigtable, an attempt to apply a mutation with
an empty row key leads to the following error being produced

```
unable to apply: rpc error: code = InvalidArgument desc = Row keys must be non-empty
```

bttest does not conform to this behaviour. This commit:

- adds error-handling when given a mutation with an empty row key, and
- adds a regression test.

grpc.WithInsecure is deprecated. It was marked as such by v1.43.0 of the
library. See here:

grpc/grpc-go#4718

This commit removes a reference to this from the example comment.