Prevent "nil does not understand keys" error in Ndef completion search

[jamshark70]

The line ^this.all[Server.default.name].keys assumes that every server object has a ProxySpace repository. This isn't true: the repository is created with the first Ndef for that server. So your first attempt to use Ndef in a session will produce a "'keys' not understood" error. This PR guards against this.

(I haven't considered the "@TODO" -- I'm not changing the spec, just fixing a detail that made it not work according to the spec.)

ERROR: Message 'keys' not understood.
RECEIVER:
   nil
ARGS:
PATH: /home/dlm/.config/SuperCollider/startup.scd

PROTECTED CALL STACK:
    Meta_MethodError:new    0x55e03950f880
        arg this = DoesNotUnderstandError
        arg what = nil
        arg receiver = nil
    Meta_DoesNotUnderstandError:new 0x55e039511bc0
        arg this = DoesNotUnderstandError
        arg receiver = nil
        arg selector = keys
        arg args = []
    Object:doesNotUnderstand    0x55e03864d5c0
        arg this = nil
        arg selector = keys
        arg args = nil
    a FunctionDef   0x55e03bcc6a00
        sourceCode = "{
                |prefixClass, trigger, completion, provideCompletionsFunc|
                var results, defNames;

                defNames = prefixClass.prGetNames.asArray.sort;
                defNames = defNames.collect({ |name| \"\\\\\" ++ name.asString });

                Log('LanguageServer.quark').info(\"Starting with defs: %\", defNames);

                // defNames = defNames.select({
                //  |name|
                //  ...etc..."
        arg prefixClass = Ndef
        arg trigger = (
        arg completion = )
        arg provideCompletionsFunc = a Function
        var results = nil
        var defNames = nil
    LSPCompletionHandler:handle 0x55e03b6ab740
        arg this = a LSPCompletionHandler
        arg prefix = Ndef
        arg trigger = (
        arg completion = )
        var deferredResult = Deferred(952455718)
... snip...

[Error - 08:56:39] Request textDocument/completion failed.
  Message: DoesNotUnderstandError
  Code: -1943623135