[IR] Fixed a problem with signature search

The function findDescriptorBySignature should return null if no such signature has been found
letting the caller to handle this correctly. Fixes https://youtrack.jetbrains.com/issue/KT-50976

compiler/ir/serialization.common/src/org/jetbrains/kotlin/backend/common/serialization/DescriptorByIdSignatureFinderImpl.kt

@@ -151,13 +151,7 @@ class DescriptorByIdSignatureFinderImpl(
         }
         val candidates = acc
 
-        return when (candidates.size) {
+        return candidates.singleOrNull() ?: findDescriptorByHash(candidates, signature.id)
-            1 -> candidates.first()
-            else -> {
-                findDescriptorByHash(candidates, signature.id)
-                    ?: error("No descriptor found for $signature")
-            }
-        }
     }
 
     private fun findDescriptorByHash(candidates: Collection<DeclarationDescriptor>, id: Long?): DeclarationDescriptor? =