fix: Make span_to_lines return at least one line

[Muscraft]

While running rustc's lintdoc tests, I encountered a corner case with SourceMap::span_to_lines, where it would return no LineInfo when it was passed the range 0..0, which ultimately caused a panic down the line. To address this, I made it so span_to_lines would only stop including lines when the range end was less than a LineInfo's start_byte (end < line_info.start_byte). This seems to be the correct fix, as we use its negation (end >= line_info.start_byte) when determining what line a byte falls on.