fix: ensure existence of urn when calling BaseEntityResource::get

[jsdonn]

motivation: calling get() with an empty, non-null aspectNames parameter (i.e. []) results in a 404, even if the entity exists. the issue stems from the call to getInternalNonEmpty which filters out urn/aspect mappings where the value is empty (which it will be when aspectNames is empty). this results in calling get(0) on an empty map, which results in a 404.

the intended behavior should be:
urn doesn't exist - 404
urn exists -
* null aspects param - return all aspects
* empty aspects param - return base response with no aspects (i.e. only non-aspect fields)
this change checks that the urn exists in the db before calling one of the getInternal methods. additionally, because we do this exists check, we can now use getInternal instead of getInternalNonEmpty to get our intended behavior.

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
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