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')

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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.