Sources with no indented lines should be disregarded in bundle auto-indent

[Rich-Harris]

Currently, a source that doesn't have any indented lines will be given the default indentStr of \t. When bundling several sources, tabs will therefore be over-represented - so in this situation...

// foo.js
export default 'this is foo';

// bar.js
export default 'this is bar';

// main.js
import foo from 'foo';
import bar from 'bar';

function logFoo () {
  console.log( foo );
}

function logBar () {
  console.log( bar );
}

...the bundle will incorrectly assume that tabs should be used, instead of two spaces