Fix(admin-ui): Fix error if no array of assets is provided

[Spielboerg]

If no assets array is set via the assets input of the ProductAssetsComponent this error is thrown:

ERROR TypeError: Cannot read properties of undefined (reading 'slice')
    at ProductAssetsComponent.set assetsSetter [as assetsSetter] (vendure-admin-ui-catalog.js:7092)
    at setInputsForProperty (core.js:10960)
    at elementPropertyInternal (core.js:10004)
    at Module.ɵɵproperty (core.js:14787)
    at RecipeDetailComponent_Template (recipe-detail.component.html:61)
    at executeTemplate (core.js:9599)
    at refreshView (core.js:9465)
    at refreshComponent (core.js:10636)
    at refreshChildComponents (core.js:9261)
    at refreshView (core.js:9515)

This pull request fixes this by using an empty array as a fallback.

I can change it to this alternative and more robust implementation if you like this solution better:

this.assets = Array.isArray(val) ? val.slice() : [];

[michaelbromley]

Thanks for the fix!