K2: Fix false-positive RETURN_TYPE_MISMATCH_ON_OVERRIDE

The reason was that `substitutorByMap` ignored the difference between
`T` and `T?`

^KT-57001 Fixed

compiler/testData/diagnostics/tests/generics/nullableTypeParameterScope.kt

@@ -0,0 +1,11 @@
+// FIR_IDENTICAL
+// ISSUE: KT-57001
+
+interface ConverterFromString<T> {
+
+    fun ofS(s: String): T
+
+    fun nullable(nullText: String): ConverterFromString<T?> = object: ConverterFromString<T?> {
+        override fun ofS(s: String): T? = if (s == nullText) null else this@ConverterFromString.ofS(s)
+    }
+}