Allow EOF IO passed to JSON::PullParser.new

[Blacksmoke16]

EOF is a valid pull parser token type. Because of this, it makes sense to set the initial kind to EOF if that's the initial state of the input versus raising an exception.

Discovered via Blacksmoke16/oq#82

[Blacksmoke16]

This is good for another review @asterite.

[asterite]

Looks good. On second thought, what's the use case for this? An empty json is invalid. I think json pull parser only produces valid json expressions, and eof is not one of them.

[Blacksmoke16]

@asterite The use case is for oq in the case where the jq filter doesn't output anything. E.g. jq '.[] | select(.name != "foo")' <<<$'[{"name":"foo"}]'. When using a filter like this with oq, and with the output format specified to something other than JSON, the pull parser is failing because the IO is already essentially empty, which is this line: https://github.com/Blacksmoke16/oq/blob/master/src/converters/yaml.cr#L11

oq -i yaml -o yaml '.[] | select(.name != "foo")' <<<$'- name: foo'
oq error: Unexpected token: <EOF> at line 1, column 1

The solution in oq land is of course to just return early if there is no data to process. However I don't think there's a way to know that the related IO has no data, so this seemed like the most logical solution. I suppose I could just rescue this exception but 🤷.

[straight-shoota]

@asterite This change does not parse an empty document as valid. It just makes sure to properly forward the information that it has reached EOF on the input stream.

[asterite]

Thanks! Feel free to merge.

[straight-shoota]

I'm actually unsure if this qualifies as a bug-fix or should wait for the end of the feature freeze.
(Thumbs up for bug fix, thumbs down for wait)

[beta-ziliani]

I need an emoji for a horizontal thumb :-D really, both interpretations are valid here. I vote for merging it. BTW @Blacksmoke16 thanks for refactoring the case, that's also a nice improvement.

[straight-shoota]

I think I'd rather postpone this to 1.2. We're about to release 1.1 and there's an off chance that this might have any unforeseeable effect. It's not an urgent bug fix and easy to work around.