Fix concatenation of null to a warning message

[gaearon]

Thanks to the renaming in #13165, I noticed a bunch of cases where we could accidentally concatenate null to a warning message if the stack was missing.

I changed getCurrentFiberStackInDevOrNull to always return a string, and renamed it to getCurrentFiberStackInDev. This fixed existing callsites that already relied on it being a string (which wasn't always the case). Then I removed a few || '' that now became unnecessary.

I also changed the "isomorphic" type of ReactDebugCurrentFrame.getStackAddendum() to be tighter since we know both SSR and Fiber implementation return a string now. This is a bit optimistic because in practice ReactDebugCurrentFrame could be from an old React version that didn't have the || '' isomorphic fallback, and its getStackAddendum could be pointing to another renderer that has a version before this change (e.g. maybe something like React 16.4 + React DOM 16.5 + ReactDOMServer 16.4). In that case you could potentially see the null bug in the warning message. I don't think this matters because (a) we recommend to keep versions in sync, (b) we already have this bug in a bunch of places, and I'd prefer tightening it up now than worrying about what happens in an edge case where you mix past and present versions.

[gaearon]

(Why didn't tests catch this? It's not that easy to get no stack at all. You either have to reproduce an existing bug, the kind that #13158 partially fixes, or forget to update the current Fiber pointer, which could also happen if there's a bug.)

[trueadm]

Seems reasonable enough and a nice way to fix the issue. Nice one.