Added new method on IExternalRule to help with invariantexpression ch… (#2725)

On an effort to remove null rule binding, we need the ability to have an
option to apply null binding before doing the check if binding is null.
This change adds a new method on IExternalRule to help with that.

src/libraries/Microsoft.PowerFx.Core/App/Controls/IExternalRule.cs

@@ -22,5 +22,8 @@ namespace Microsoft.PowerFx.Core.App.Controls
 
         // Returns true when Binding is non-null, otherwise false.
         bool HasValidBinding { get; }
+
+        // Returns true when rule is constant.
+        bool IsInvariantExpression { get; }
     }
 }

src/libraries/Microsoft.PowerFx.Core/Binding/Binder.cs

@@ -3619,9 +3619,7 @@ namespace Microsoft.PowerFx.Core.Binding
                     // we need to mark the node as constant, and save the control info so we may look up the
                     // rule later.
                     if (controlInfo?.GetRule(property.InvariantName) is IExternalRule rule &&
-                        rule.HasValidBinding &&
-                        !rule.HasErrorsOrWarnings &&
-                        rule.Binding.IsConstant(rule.Binding.Top))
+                        rule.IsInvariantExpression)
                     {
                         value = controlInfo;
                         isConstant = true;