Fix null character line

[complexusprada]

If a line contains a null character (^@), it breaks the following functions:

vim.fn.strdisplaywidth(line_context.line)

end_col = vim.fn.byteidx(cur_line, vim.fn.charidx(cur_line, cur_col) + 1),

added an early return if a line contains blob

[complexusprada]

null_character.txt
file with null characters