Use UNKNOWN_LOCATION if node location does not exist

[ianyong]

This change is necessary for multiple file programs to work correctly in the frontend. This is because the AST nodes which are added by the preprocessor do not have a corresponding location (due to not being parsed from code in the first place). One of the instances of .loc! is being called (no idea which because it's almost impossible to trace without proper error handling), resulting in the evaluation of multiple-file programs dying.

We really should ban the use of the non-null assertion operator (!). Its only purpose is to let lazy programmers write code that bypass the type checker.

Part of source-academy/frontend#2176.

[coveralls]

Pull Request Test Coverage Report for Build 4523966829

• 107 of 111 (96.4%) changed or added relevant lines in 34 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 83.517%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/interpreter/interpreter-non-det.ts	3	4	75.0%
src/parser/source/rules/noEval.ts	1	2	50.0%
src/typeChecker/internalTypeErrors.ts	2	3	66.67%
src/utils/operators.ts	2	3	66.67%

Totals
Change from base Build 4496667413:	-0.1%
Covered Lines:	10225
Relevant Lines:	11863

---

💛 - Coveralls

[zhaojj2209]

LGTM