jinja: support none|string (#18995)
* jinja: support none|string
* Update common/jinja/value.cpp
Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
* Update tests/test-jinja.cpp
Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
* Add as_string()
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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>