Fix empty strings being treated as URLs

[ivanistuk]

Adds a check to see if text is empty before making it a link.

Fixes #279.

[ImRodry]

While this works this isn't the optimal solution in my opinion. I believe you should remove the last ? when adding relativePath on this line

json-viewer/extension/src/json-viewer/url-pattern.js

Line 73 in 4195c33

module.exports = new RegExp("^(" + absolutePath() + "|" + relativePath() + ")$", "i");