Fix NullRefEx bug when accessing scriptFile.ReferencedFiles

This happens because the ScriptFile ctor does not initialize all its
public props.  I added initialization for the other public props
except for the ScriptAst prop.  I don't see an empty Ast.  Perhaps null
is OK for this prop?

This address vscode-powershell bug PowerShell/vscode-powershell#1675

Also, for the 2.0.0 branch, we should see if we can use Array.Empty<>()
for initialization.  It isn't availble to net45x.  :-(

src/PowerShellEditorServices/Workspace/ScriptFile.cs

@@ -26,7 +26,6 @@ public class ScriptFile
             "\n"
         };
 
-        private Token[] scriptTokens;
         private Version powerShellVersion;
 
         #endregion
@@ -116,7 +115,8 @@ public ScriptBlockAst ScriptAst
         /// </summary>
         public Token[] ScriptTokens
         {
-            get { return this.scriptTokens; }
+            get;
+            private set;
         }
 
         /// <summary>
@@ -146,11 +146,16 @@ public ScriptFile(
             TextReader textReader,
             Version powerShellVersion)
         {
+            this.powerShellVersion = powerShellVersion;
+
             this.FilePath = filePath;
             this.ClientFilePath = clientFilePath;
             this.IsAnalysisEnabled = true;
             this.IsInMemory = Workspace.IsPathInMemory(filePath);
-            this.powerShellVersion = powerShellVersion;
+            this.ReferencedFiles = new string[0];
+            this.SyntaxMarkers = new ScriptFileMarker[0];
+            this.FileLines = new List<string>();
+            this.ScriptTokens = new Token[0];
 
             this.SetFileContents(textReader.ReadToEnd());
         }
@@ -600,6 +605,8 @@ private void ParseFileContents()
             try
             {
 #if PowerShellv5r2
+                Token[] scriptTokens;
+
                 // This overload appeared with Windows 10 Update 1
                 if (this.powerShellVersion.Major >= 5 &&
                     this.powerShellVersion.Build >= 10586)
@@ -610,17 +617,19 @@ private void ParseFileContents()
                         Parser.ParseInput(
                             this.Contents,
                             this.FilePath,
-                            out this.scriptTokens,
+                            out scriptTokens,
                             out parseErrors);
                 }
                 else
                 {
                     this.ScriptAst =
                         Parser.ParseInput(
                             this.Contents,
-                            out this.scriptTokens,
+                            out scriptTokens,
                             out parseErrors);
                 }
+
+                this.ScriptTokens = scriptTokens;
 #else
                 this.ScriptAst =
                     Parser.ParseInput(
@@ -638,7 +647,7 @@ private void ParseFileContents()
                         ex.Message);
 
                 parseErrors = new[] { parseError };
-                this.scriptTokens = new Token[0];
+                this.ScriptTokens = new Token[0];
                 this.ScriptAst = null;
             }