Notice: Uninitialized string offset: 0 in Drupal\Core\Asset\LibraryDiscoveryParser->buildByExtension()

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As a bug will need a test case.

Tried replicating with devel installed but didn't see this log.

This occurs when the `version` is an empty string. PHP's `is_string()` returns TRUE on `''` strings.

$str = '';
$result = is_string($str);
$char = $str[0]; // Uninitialized string offset: 0
echo "Char: $char";

It's not really a bug, as it is an oversight into how `is_string()` acts on empty strings. Instead, possibly we could use strlen()?

Here is the patch.

Correct patch