Improve Resource.Merge for the empty values case

[carlosalberto]

Follow up to #1345 (comment)

Consider amending the following Resource.Merge section:

If a key exists on both the old and updating resource, the value of the updating
resource MUST be picked (even if the updated value is empty).

Probably not that important, as Resource.Merge will quite often be called only at application start time (besides being a very cheap operation).

cc @bogdandrutu

[Oberon00]

I think overriding a value with an empty string can also be a feature. Also, where do we expect these empty strings to come from? Wouldn't it usually be better to tweak the producer to not produce the resource attribute at all instead of an empty string?