FIR checker: COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT

FIR simply ignores anything after the first comma if the when expression
doesn't have a subject. Hence, the checker has to rely on PSI structure
instead.

compiler/testData/diagnostics/tests/smartCasts/kt6819.fir.kt

@@ -1,7 +1,7 @@
 public fun test(o: String?): Boolean {
     return when {
         // Data flow info should propagate from o == null to o.length
-        o == null, o.length == 0 -> false
+        o == null<!COMMA_IN_WHEN_CONDITION_WITHOUT_ARGUMENT!>,<!> o.length == 0 -> false
         else -> true
     }
-}
+}