fu: CFG cond context for extension method calls

Author: aschackmull

csharp/ql/lib/semmle/code/csharp/controlflow/ControlFlowGraph.qll

@@ -345,7 +345,9 @@ private module Input implements InputSig1, InputSig2 {
 
   predicate inConditionalContext(Ast::AstNode n, ConditionKind kind) {
     kind.isNullness() and
-    exists(QualifiableExpr qe | n = qe.getQualifier() and qe.isConditional())
+    exists(QualifiableExpr qe | qe.isConditional() |
+      n = qe.getQualifier() or qe.(ExtensionMethodCall).getArgument(0) = n
+    )
   }
 
   predicate postOrInOrder(Ast::AstNode n) { n instanceof YieldBreakStmt or n instanceof Call }