MapRowDataSnapshot throw when accessing absent column

[dmfs]

At present, accessing an absent column of a MapRowDataSnapshot merely returns an absent value, just like when the value was null.
Accessing a row which isn't in the result, however, usually indicates an error of not providing the right projection when doing a query.
We should check if there are any valid cases of accessing an absent row and otherwise handle this case separately from null values and throw instead.