fix: Coherence crashes if slvProduct is null

[vincentvaroquauxads]

To fix the “software regression” (commit: 7759322509 (“update StackUtils and add test”, 2021-04-08) on snap-engine)

See discussion : https://forum.step.esa.int/t/software-regression-nodeid-coherence-slvproductname-is-null/34286

It’s a little bit ugly, but it works.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}