Fix a null-ref exception in DynamicHelpImpl (#2292)

PSReadLine/DynamicHelp.cs

@@ -151,6 +151,9 @@ namespace Microsoft.PowerShell
             string commandName = null;
             string parameterName = null;
 
+            // Simply return if nothing is rendered yet.
+            if (_singleton._tokens == null) { return; }
+
             foreach(var token in _singleton._tokens)
             {
                 var extent = token.Extent;

test/DynamicHelpTest.cs

@@ -97,6 +97,13 @@ PARAMETERS
             return paramHelp;
         }
 
+        [SkippableFact]
+        public void DynHelp_GetFullHelp_OnEmptyLine()
+        {
+            TestSetup(KeyMode.Cmd);
+            Test("", Keys(_.F1, _.Enter));
+        }
+
         [SkippableFact]
         public void DynHelp_GetFullHelp()
         {
@@ -108,6 +115,13 @@ PARAMETERS
                 ));
         }
 
+        [SkippableFact]
+        public void DynHelp_GetParameterHelp_OnEmptyLine()
+        {
+            TestSetup(KeyMode.Cmd);
+            Test("", Keys(_.Alt_h, _.Enter));
+        }
+
         [SkippableFact]
         public void DynHelp_GetParameterHelp_And_Clear()
         {