Join Lines: join initializer + if null check to elvis (KT-22388)

#KT-22388 Fixed

idea/testData/joinLines/initializerAndIfToElvis/simple.kt

@@ -0,0 +1,5 @@
+fun foo(list: List<String>): Int {
+    <caret>val first = list.firstOrNull()
+    if (first == null) return -1
+    return first.length
+}

idea/testData/joinLines/initializerAndIfToElvis/simple.kt.after

@@ -0,0 +1,4 @@
+fun foo(list: List<String>): Int {
+    val first = <caret>list.firstOrNull() ?: return -1
+    return first.length
+}