Avoid empty node in empty global scope (microsoft#1226)

* Add null checks

* Avoid null node in empty global scope

* Remove unrelated change

Author: Mikhail Arkhipov

src/Analysis/Ast/Impl/Extensions/ScopeExtensions.cs

@@ -27,6 +27,8 @@ public static int GetBodyStartIndex(this IScope scope) {
                     return cd.HeaderIndex;
                 case FunctionDefinition fd:
                     return fd.HeaderIndex;
+                case null:
+                    return 0;
                 default:
                     return scope.Node.StartIndex;
             }

src/Analysis/Ast/Impl/Values/Scope.cs

@@ -126,9 +126,9 @@ public EmptyGlobalScope(IPythonModule module) {
         }
         public IPythonModule Module { get; }
         public string Name => string.Empty;
-        public ScopeStatement Node => null;
+        public ScopeStatement Node => Module.Analysis.Ast;
         public IScope OuterScope => null;
-        public IGlobalScope GlobalScope { get; protected set; }
+        public IGlobalScope GlobalScope { get; }
         public IReadOnlyList<IScope> Children => Array.Empty<IScope>();
         public IEnumerable<IScope> EnumerateTowardsGlobal => Enumerable.Repeat(this, 1);
         public IEnumerable<IScope> EnumerateFromGlobal => Enumerable.Repeat(this, 1);