Review behaviour when WATCH_NAMESPACE is missing or an empty string

[squaremo]

From #278:

WATCH_NAMESPACE	outcome	expected?
missing	crashloop with log message about requiring WATCH_NAMESPACE	unclear, I would expect it to watch all namespaces -- is this an operator-framework convention?
""	cannot list resource "stacks" in API group "pulumi.com" at the cluster scope	unclear, I would expect that it watches all namespaces; maybe this is another operator-framework convention. In any case, I would not expect it to attempt to watch cluster-level resources (but perhaps I'm misinterpreting the error)

My expectation for both these scenarios is that all namespaces would be watched (and this seems to be the intention of operator-sdk, too: https://sdk.operatorframework.io/docs/building-operators/golang/operator-scope/).