fix(coverage): do not generate script coverage with empty url

[kt3k]

closes #24004

This PR skips saving script_coverage as json when script_coverage.url is an empty string. This happens (strangely) when the test case includes Deno.statSync call (See #24004 for details).

[bartlomieju]

You get empty URL because the struct used in statSync is code generated:

deno/ext/fs/30_fs.js

Line 300 in a0ddf73

function createByteStruct(types) {

[kt3k]

You get empty URL because the struct used in statSync is code generated:

Thanks! That explains it!