ignore_changes changing a null map to empty

Fix an error where using `ignore_changes` would cause some null maps to
be saved as an empty map.

internal/terraform/node_resource_abstract_instance.go

@@ -1289,10 +1289,15 @@ func processIgnoreChangesIndividual(prior, config cty.Value, ignoreChangesPath [
 		}
 
 		var newVal cty.Value
-		if len(configMap) == 0 {
-			newVal = cty.MapValEmpty(v.Type().ElementType())
-		} else {
+		switch {
+		case len(configMap) > 0:
 			newVal = cty.MapVal(configMap)
+		case v.IsNull():
+			// if the config value was null, and no values remain in the map,
+			// reset the value to null.
+			newVal = v
+		default:
+			newVal = cty.MapValEmpty(v.Type().ElementType())
 		}
 
 		if len(vMarks) > 0 {