Single quoted strings containing whitespace are parsed as empty strings

Take current grammar and try to parse SELECT ' ' from t using pest editor.

Result:

- Select
  - Projection > Column > String: ""
  - Scan > Table: "t"
- EOF > EOI: ""

But the string should be a whitespace:

- Select
  - Projection > Column > String: " "
  - Scan > Table: "t"
- EOF > EOI: ""