fix: Ensure no error is thrown if there are no sources in a mapping

[martin-fleck-at]

Fixes #143

[github-actions]

Unit Test Results

43 tests  ±0   43 ✅ ±0   44s ⏱️ -1s
 9 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit c66e7c1. ± Comparison against base commit a2943e8.

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[harmen-xb]

@martin-fleck-at I still see the issue.

When I have an entity with one or more attributes and I right click on the entity and choose 'Derive mapping' I get a mapping where the target attributes are already listed:

mapping:
    id: NewMapping2
    target:
        entity: Address
        mappings:
            - attribute: Customer_Id
            - attribute: Street
            - attribute: CountryCode

But if I right click on the model and choose 'New element > Mapping' and choose the same target entity the target attributes are not in the file (and the mapping diagram is not rendered correctly):

mapping:
    id: NewMapping
    target:
        entity: Address

I think it's best to solve this issue as part of #147, since mappings without listing the target attributes is already working there. As I mentioned in the other PR, I would rather not have the target attributes listed if there is no mapping on it.

[martin-fleck-at]

Sure, I'll have a look at this and merge it with the PR in #147