C#: Adjust the location of the empty element.

Author: michaelnebel

csharp/ql/lib/semmle/code/csharp/Location.qll

@@ -74,8 +74,20 @@ class Location extends @location {
 }
 
 /** An empty location. */
-class EmptyLocation extends Location {
-  EmptyLocation() { this.hasLocationInfo("", -1, -1, -1, -1) }
+class EmptyLocation extends SourceLocation {
+  EmptyLocation() { locations_default(this, _, -1, -1, -1, -1) }
+
+  override predicate hasLocationInfo(
+    string filepath, int startline, int startcolumn, int endline, int endcolumn
+  ) {
+    filepath = "" and
+    startline = 0 and
+    startcolumn = 0 and
+    endline = 0 and
+    endcolumn = 0
+  }
+
+  override string toString() { result = "empty location" }
 }
 
 /**