fix: check value is a string in isSimpleTermDefinitionPrefix

[jeswr]

Fixes the spec test error seen in rubensworks/jsonld-streaming-parser.js#113.

@rubensworks note that an observation here is that since #65 we are now normalizing

{
  "@version": 1.1,
  "@protected": true,
  "term": null
}

as

{
  "@version": 1.1,
  "term": {
    "@id": null,
    "@protected": true,
  }
}

which I'm not sure is correct...

[coveralls]

Pull Request Test Coverage Report for Build 6488780797

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 6478874410:	0.0%
Covered Lines:	543
Relevant Lines:	543

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💛 - Coveralls

[rubensworks]

Thanks!

[rubensworks]

Released as 2.3.3.