C#: Fix BDD limit issue (thank you @jbj).

Author: michaelnebel

csharp/ql/lib/semmle/code/dotnet/Element.qll

@@ -43,8 +43,15 @@ class Element extends @dotnet_element {
 
   /**
    * Gets a comma-separated list of the names of the primary CodeQL classes to which this element belongs.
+   *
+   * If no primary class can be determined, the result is `"???"`.
    */
-  final string getPrimaryQlClasses() { result = concat(this.getAPrimaryQlClass(), ",") }
+  final string getPrimaryQlClasses() {
+    result = strictconcat(this.getAPrimaryQlClass(), ",")
+    or
+    not exists(this.getAPrimaryQlClass()) and
+    result = "???"
+  }
 
   /**
    * Gets the name of a primary CodeQL class to which this element belongs.
@@ -53,11 +60,12 @@ class Element extends @dotnet_element {
    * which they belong; for example, `AddExpr` is a primary class, but
    * `BinaryOperation` is not.
    *
-   * This predicate always has a result. If no primary class can be
-   * determined, the result is `"???"`. If multiple primary classes match,
-   * this predicate can have multiple results.
+   * If no primary classes match, this predicate has no result. If multiple
+   * primary classes match, this predicate can have multiple results.
+   *
+   * See also `getPrimaryQlClasses`, which is better to use in most cases.
    */
-  string getAPrimaryQlClass() { result = "???" }
+  string getAPrimaryQlClass() { none() }
 }
 
 /** An element that has a name. */