Previously calling `script.code()` would fail if the script didn't have
a `name`. This wasn't being hit in practice because the only place that
constructs GreasemonkeyScripts was checking for that condition and add
the filename there as a fallback.
This change make the `name` attribute more explicitly mandatory by
failing with a `ValueError` if it is not provided and make it still
possible to use the filename fallback in that case by adding a
`filename` keyward argument to `__init__()`.
Additionally where `script_meta` is used in `script.code()` a fallback
to and emptry string was added so it doesn't fail for raw javascript
files without greasemonkey metadata.
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