[NI] Remove lambda coersion to Unit in case of error return type(s)

Coersion to Unit from error type leads to misleading type mismatches:
"expected <expected lambda return type> found Unit", despite no user-provided Unit / empty lambda.
These diagnostics were collected, but not reported before, and that had been disguising the issue for a while.

KT-34729 Fixed

compiler/testData/diagnostics/tests/inference/kt28654.kt

@@ -2,7 +2,7 @@
 // !DIAGNOSTICS: -UNUSED_VARIABLE
 // Related issue: KT-28654
 
-fun <K> select(): K = <!NI;TYPE_MISMATCH, OI;TYPE_INFERENCE_EXPECTED_TYPE_MISMATCH!>run { <!OI;TYPE_MISMATCH!><!>}<!>
+fun <K> select(): K = <!NI;TYPE_MISMATCH, OI;TYPE_INFERENCE_EXPECTED_TYPE_MISMATCH!>run <!NI;TYPE_MISMATCH!>{ <!OI;TYPE_MISMATCH!><!>}<!><!>
 
 fun test() {
     val x: Int = select()