clear_annotations function fails

[kevinjardine]

clear_annotations is failing to actually delete all annotations when you do not specify a name.

This is because of the line:

$name = get_metastring_id($name);

which maps even empty strings to a metadata id, assuming that there is an empty string in the metastrings table, which is usually the case.

The result is that the function attempts to delete annotations which have no name, instead of deleting all annotations as intended.

[ben]

(In [svn:2134]) Clear annotations now clears annotations properly when no name is specified. Fixes #357