[FIR2IR] Make safe call result always nullable

compiler/testData/ir/irText/expressions/safeCalls.fir.txt

@@ -158,10 +158,10 @@ FILE fqName:<root> fileName:/safeCalls.kt
   FUN name:box visibility:public modality:FINAL <> () returnType:kotlin.Unit
     BLOCK_BODY
       TYPE_OP type=kotlin.Unit origin=IMPLICIT_COERCION_TO_UNIT typeOperand=kotlin.Unit
-        BLOCK type=kotlin.Int origin=SAFE_CALL
+        BLOCK type=kotlin.Int? origin=SAFE_CALL
           VAR IR_TEMPORARY_VARIABLE name:tmp_5 type:kotlin.Int [val]
             CONST Int type=kotlin.Int value=42
-          WHEN type=kotlin.Int origin=null
+          WHEN type=kotlin.Int? origin=null
             BRANCH
               if: CALL 'public final fun EQEQ (arg0: kotlin.Any?, arg1: kotlin.Any?): kotlin.Boolean declared in kotlin.internal.ir' type=kotlin.Boolean origin=EQEQ
                 arg0: GET_VAR 'val tmp_5: kotlin.Int [val] declared in <root>.box' type=kotlin.Int origin=null