FIR2IR: never generate empty when #KT-55459 Fixed

In details, this commit changes the following:
- it converts FIR when without branches to empty IR block without when
- it doesn't drop empty else branches in when anymore

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

@@ -48,6 +48,9 @@ FILE fqName:<root> fileName:/ifElseIf.kt
         BRANCH
           if: GET_VAR 'flag: kotlin.Boolean declared in <root>.testEmptyBranches2' type=kotlin.Boolean origin=null
           then: CONST Boolean type=kotlin.Boolean value=true
+        BRANCH
+          if: CONST Boolean type=kotlin.Boolean value=true
+          then: BLOCK type=kotlin.Unit origin=null
   FUN name:testEmptyBranches3 visibility:public modality:FINAL <> (flag:kotlin.Boolean) returnType:kotlin.Unit
     VALUE_PARAMETER name:flag index:0 type:kotlin.Boolean
     BLOCK_BODY