Fix error reading array that could be undefined in ows service

[ianwallen]

Fix error reading array that could be undefined in ows service

Without this fix the following error could be displayed in JavaScript console Cannot read properties of undefined (reading 'length')

Checklist

• [x] I have read the contribution guidelines
• [x] Pull request provided for main branch, backports managed with label
• [ ] Good housekeeping of code, cleaning up comments, tests, and documentation
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[josegar74]

@ianwallen do you have a service that cause the issue and can be used for testing?

[ianwallen]

While I was testing the file uploads for #8427 in our environment which was based on GN 4.2.9 branch, I was seeing the error constantly showing up in the java console. I assumed that it was good practice to add a if (Array.isArray(layers)) { before using the array.
If it should never happen that layers is not an array and it is always defined then we can abandon this PR and I can raise a bug the next time I see the error occur.