JVM_IR: Fix codegeneration for missing branches.

This used to generate an ACONST_NULL instruction leading to incorrect
stack height for code such as:

```
if (condition) else 32
```

compiler/testData/codegen/bytecodeText/boxingOptimization/maxMinBy.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 // FILE: list.kt
 
 val intList = listOf(1, 2, 3)