fix: avoid 'Expression is always true' error

[Rindrics]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

  • Bug fix
    • This PR fixes 'Expression is always true' error when executing yarn test

• What is the current behavior? (You can also link to an open issue here)

  • ref: #48

• What is the new behavior (if this is a feature change)?

  • This change allows the test to pass without 'Expression is always true' error

• Other information:

  • The another approach to avoid the error is to suppress 'strict-type-predicates' error:

# tslist.json
 {
-  "extends": "tslint-config-standard"
+  "extends": "tslint-config-standard",
+  "rules": {
+    "strict-type-predicates": false
+  }

but I rejected the method because it could cause other important errors to be missed