Fixed bug where null urlTemplate caused exception in TileLayer construction

[JaffaKetchup]

It's not necessarily safe to assume a URL template will be provided when WMS options are missing.
Some plugins, for example, (*cough *cough FMTC) use TileLayer to transport other useful information, but not the URL itself.

[mootw]

sure, though is it bad practice whatever fmtc is doing?? (i don't want to encourage bad practice by allowing something that should be avoided)

[JaffaKetchup]

Well, it might be bad practise in some people's books, but it works just fine. If it is bad practise, then it needs a proper assertion. Maybe we also need to think about whether how we're expressing it is the best way - but I can't think of any alternatives off the top of my head.

One example is converting object A to B, which requires a TileLayer. B has two methods. One requires the TileLayer to have a URL, one doesn't care what's inside the TileLayer (but B must still have it), IYSWIM.
EDIT: just realised how nonsensical that is - it's a little hard to explain. But resolving it would require having a nullable field then enforcing via an assert. Maybe I should do that anyway.

Also used to access the default templating function, but that needs to change now I guess.