[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/testsWithStdLib/resolve/hidesMembers2.fir.kt

@@ -1,5 +1,7 @@
 // !CHECK_TYPE
 // !DIAGNOSTICS: -UNUSED_EXPRESSION -UNUSED_PARAMETER -UNUSED_VARIABLE
+// !WITH_NEW_INFERENCE
+
 class A {
     fun forEach() = this
     fun forEach(i: Int) = this