fix #3056

[sramazzina]

fix #3056 NullPointerException on pipelines with transforms receiving more than one flow in input

The following considerations applies to this PR:

1. This fix start from the assumption when we arrive to a transform, we arrive there because we have a flow that takes us there otherwise we never arrive there.
2. We definitely verified that the problem arises because, at the time the issue is raised, the inputRowMeta reference is null
3. The place where this change acts is the only place where, in the engine, the inputRowMeta reference is set during the lifecycle of the transform
4. If we have more than one flow that is entering a transform, the change keeps only the RowMeta references that are valid and we suppose that at least one of these must be valid because of the assumption made at point 1 above

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