LE Audio: Scan Delegator shall clear metadata on meta_len = 0

[Thalley]

Describe the bug
In the scan_delegator_mod_src we have the following

		/* If the metadata len is 0, we shall not overwrite the existing metadata */
		if (subgroups[i].metadata_len == 0) {
			continue;
		}

Which seems to be a leftover of a previous version of the BASS spec, as the version 1.0 does not state anything like that.

To Reproduce
N/A

Expected behavior
If the broadcast assistant sets the metadata_len to 0, we should clear the metadata instead.

Impact
Missing spec compliance

Logs and console output
N/A

Environment (please complete the following information):

• Commit SHA or Version used: ee451e5

Additional context
N/A