fix(dynamodb-local): fix check on inMemory to allow setting dbPath

[attilaersek]

What changed?
Adjusted the check in the dynamodb-local service so that the inMemory flag and dbPath setting are evaluated correctly.
Added appropriate test(s) to cover both modes and ensure dbPath can be set when inMemory = false.

[shivaraj-bh]

Not a concern of this PR, but it occurred to me while reviewing that: asserting in inMemory option with (dbPath) cross-option reference felt a bit awkward. Maybe a NixOS module style assertions option (example) would be nice in such cases. Though straightforward, not enough cases to add support yet; I will re-visit later.

[shivaraj-bh]

Not a concern of this PR, but it occurred to me while reviewing that: asserting in inMemory option with (dbPath) cross-option reference felt a bit awkward. Maybe a NixOS module style assertions option (example) would be nice in such cases. Though straightforward, not enough cases to add support yet; I will re-visit later.

(Created #660 to track)