[Sema] Fix crash when diagnosing ambiguous trailing closure inits

Fixes #85376.

This fixes a compiler crash that occurred when diagnosing an ambiguous call
using trailing closure syntax, where one of the candidates was a function or
initializer with no parameters.

lib/Sema/CSDiagnostics.cpp

@@ -2142,6 +2142,8 @@ bool TrailingClosureAmbiguityFailure::diagnoseAsNote() {
       return false;
 
     const ParameterList *paramList = callee->getParameters();
+    if (paramList->getArray().empty())
+      return false;
     const ParamDecl *param = paramList->getArray().back();
 
     // Soundness-check that the trailing closure corresponds to this parameter.