fix: var value handle

[hundun000]

resolve #528
resolve #527

以及实现:

setVar:emptyText=; 预期赋值为空字符创

鉴于多处对于getValueFromState取不到都需要处理，统一包成getValueFromStateElseKey。

测试用例：

setVar:zero=0;
setVar:emptyText=;
Tester:absent var shows as {absent}, zero var shows as {zero}, empty text var shows as {emptyText}.
setVar:ref_absent=absent;
setVar:ref_zero=zero;
setVar:ref_emptyText=emptyText;
Tester:ref_absent var shows as {ref_absent}, ref_zero var shows as {ref_zero}, ref_empty text var shows as {ref_emptyText}.
setVar:computed=zero + zero;
Tester:computed zero var shows as {computed}