[NI] Avoid constraints from expected type for effectively empty system

Expression will be checked against expected type later.

 Theoretically, this is not very good, but it aligns with the old
 inference, plus it helps avoiding multiple type mismatch diagnostics.

compiler/testData/diagnostics/tests/inference/kt1293.kt

@@ -5,7 +5,7 @@ package kt1293
 
 fun main() {
     val intArray = arrayOfNulls<Int>(10)
-    val <!UNUSED_VARIABLE!>i<!> : Int = <!NI;TYPE_MISMATCH, TYPE_MISMATCH!>intArray[0]<!>
+    val <!UNUSED_VARIABLE!>i<!> : Int = <!TYPE_MISMATCH!>intArray[0]<!>
     requiresInt(<!TYPE_MISMATCH!>intArray[0]<!>)
 }