[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/objects/kt5527.kt

@@ -7,13 +7,13 @@ class A {
 }
 
 fun foo() {
-    val i1: Int = <!NI;TYPE_MISMATCH, TYPE_MISMATCH!>Boo<!>
-    val i2: Int = <!TYPE_MISMATCH!>A.<!NI;TYPE_MISMATCH!>Boo<!><!>
+    val i1: Int = <!TYPE_MISMATCH!>Boo<!>
+    val i2: Int = <!TYPE_MISMATCH!>A.Boo<!>
     useInt(<!TYPE_MISMATCH!>Boo<!>)
     useInt(<!TYPE_MISMATCH!>A.Boo<!>)
 }
 fun bar() {
-    val i1: Int = <!NI;TYPE_MISMATCH, TYPE_MISMATCH!>Unit<!>
+    val i1: Int = <!TYPE_MISMATCH!>Unit<!>
     useInt(<!TYPE_MISMATCH!>Unit<!>)
 }