fix: skip empty citation segments after semicolon split

[vnykmshr]

Fixes #348.

citation() panics when bytes.Split produces empty segments from adjacent or trailing semicolons in citation syntax (e.g. [@;], [@ref;], [@ref1;;@ref2]).

What changed

• parser/citation.go: added len(citation) == 0 check with continue after TrimSpace, before indexing into the slice
• parser/citation_test.go: added TestCitationEmptySegment covering the three panic cases

Using continue preserves valid citations on either side of an empty segment.

[kjk]

thanks!