fix: should not throw errors when navItems are undefined

[fluffynuts]

I came across an esoteric error today after updating, with the message being about not being able to parse some JSON. I eventually tracked it down by monkey-patching JSON.parse and finding the error coming from the fact that my navItems property on my component was undefined at the point at which it was being run through the JSON.parse/JSON.stringify line that I've changed.
I'm assuming the introduction of the stringify/parse is to get a deep clone of the array -- the fix supplied would treat undefined items as if there are no items.