[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/reportingImprovements/NoAmbiguityForDifferentFunctionTypes.fir.kt

@@ -1,3 +1,5 @@
+// !WITH_NEW_INFERENCE
+
 package a
 
 interface Closeable {}

compiler/testData/diagnostics/tests/inference/reportingImprovements/NoAmbiguityForDifferentFunctionTypes.kt

@@ -1,3 +1,5 @@
+// !WITH_NEW_INFERENCE
+
 package a
 
 interface Closeable {}
@@ -8,7 +10,7 @@ fun <T: Closeable, R> T.foo(block: (T)-> R) = block
 fun <T: Closeable, R> T.foo(block: (T, T)-> R) = block
 
 fun main() {
-    C().foo { // no ambiguity here
+    C().<!NI;NEW_INFERENCE_NO_INFORMATION_FOR_PARAMETER!>foo<!> { // no ambiguity here
         <!UNUSED_ANONYMOUS_PARAMETER!>www<!> ->
         <!UNRESOLVED_REFERENCE!>xs<!>
     }