gotohelm: fix map indexing and zeroOf

[chrisseto]

66671ba gotohelm: correct map indexing for non-nil zero values

Prior to this commit all map indexing operations in helm world would return
nil thanks to index's behavior under the hood. This worked for 70% of all
cases as the zero value of most map elements was zero. However, non-nil zero
values would be handled incorrectly:

map[string]string{}["missing"] // returns nil in helm

This commit updates the transpiler to appropriately detect such cases and
injects the zero value of the map's element type.

e6cc8bc gotohelm: improve special case detection in zeroOf

Prior to this commit gotohelm would incorrectly attempt to compute the zero
value of types that implemented json.{Unm,M}arshaller. In most cases, this
would lead to a transpiler crash all together. In some corner cases, it was
possible to silently generate incorrect values.

This commit improves zeroOf to detect and reject implementers of
json.{Unm,M}arshaller with a helpful error message and adds a special case
for Kubernetes' resource.Quantity type.

[chrisseto]

I'm going to merge this without k8s-operator passing because we keep hitting unrelated flakes :(