Using a mutable variable with if-elif in mixin results in unsetting the variable

[Freewalkr]

Bug Report

1. Minimal reproduce step (Required)

mixin NoCondMixin:
    _a: [str] = []
    _a += ["text1"]
    _a += ["text2"]

mixin OneCondMixin:
    _a: [str] = []
    _a += ["text1"]
    if mod == "m":
        _a += ["text2"]

mixin TwoCondMixin:
    _a: [str] = []
    _a += ["text1"]
    if mod == "m":
        _a += ["text2"]
    elif mod == "n":
        _a += ["text3"]

schema NoCondSchema:
    mixin [NoCondMixin]
    mod: str
    a: [str] = _a or ["dummy"]

schema OneCondSchema:
    mixin [OneCondMixin]
    mod: str
    a: [str] = _a or ["dummy"]

schema TwoCondSchema:
    mixin [TwoCondMixin]
    mod: str
    a: [str] = _a or ["dummy"]

ncs = NoCondSchema { mod = "m" }
ocs = OneCondSchema { mod = "m" }
tcs = TwoCondSchema { mod = "m" }

Interesting that if I don't add or ["dummy"] in TwoCondSchema, KCL throws attribute 'a' of TwoCondSchema is required and can't be None or Undefined error.

Also if I merge mixins' contents into schemas, everything works as expected.

2. What did you expect to see? (Required)

ncs:
  mod: m
  a:
  - text1
  - text2
ocs:
  mod: m
  a:
  - text1
  - text2
tcs:
  mod: m
  a:
  - text1
  - text2

3. What did you see instead (Required)

ncs:
  mod: m
  a:
  - text1
  - text2
ocs:
  mod: m
  a:
  - text1
  - text2
tcs:
  mod: m
  a:
  - dummy

4. What is your KCL components version? (Required)

KCL v0.11.0