Kotlin: Exclude operands of NotNullExpr from NullMaybe query

java/ql/src/Likely Bugs/Nullness/NullMaybe.ql

@@ -21,6 +21,8 @@ from VarAccess access, SsaSourceVariable var, string msg, Expr reason
 where
   nullDeref(var, access, msg, reason) and
   // Exclude definite nulls here, as these are covered by `NullAlways.ql`.
-  not alwaysNullDeref(var, access)
+  not alwaysNullDeref(var, access) and
+  // Kotlin enforces this already:
+  not access.getLocation().getFile().isKotlinSourceFile()
 select access, "Variable $@ may be null here " + msg + ".", var.getVariable(),
   var.getVariable().getName(), reason, "this"

java/ql/test/kotlin/query-tests/NullMaybe/NullMaybe.qlref

@@ -0,0 +1 @@
+Likely Bugs/Nullness/NullMaybe.ql

java/ql/test/kotlin/query-tests/NullMaybe/Test.kt

@@ -0,0 +1,7 @@
+fun fn(b: Boolean) {
+    var d: Double? = null
+    if (b) {
+        d = 1.0
+    }
+    println(d!!)
+}