Move blackBoxFile() testData to box/ directory

Delete all test methods (and empty test classes), since they'll be
auto-generated

compiler/testData/codegen/box/controlStructures/conditionOfEmptyIf.kt

@@ -0,0 +1,13 @@
+var result = "Fail"
+
+fun setOK(): Boolean {
+    result = "OK"
+    return true
+}
+
+fun box(): String {
+    if (setOK()) {
+    } else {
+    }
+    return result
+}