GH-2633 - Be more lenient when mapping DTOs.

[michael-simons]

We can support many more usecases of "I want to have a lightweight mapping tool" for DTOs by just
restricting the cases in which a NoRootNodeMappingException is thrown.

The idea is as follows: If the DefaultNeo4jEntityConverter does not deal with a synthesized
record comming from AggregatingMappingFunction it uses the the root map as base for mapping
if the root record is already a map value. If that is not the case and the root record does
not contain any paths that maybe aggregated later one, we synthesize a map one single time
and evaluate that for being mappable.

Closes #2633.

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