fix: allow the evaluation of null characters

[tylerablackham]

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Fix for #2059

[jaylinski]

Note: if PR is valid, also upstream this to https://github.com/handlebars-lang/handlebars-parser/blob/master/src/handlebars.l.

[tylerablackham]

@jaylinski just created an upstream PR handlebars-lang/handlebars-parser#29