Glyph guideline: name can not be set to empty string

[Just van Rossum]

There are two bugs:

1. Two guidelines, one has a name. Double-click the one with the name, empty the name: can't edit because "name has to be unique"
2. One guideline with a name, double click guideline, empty the name, click "Edit" or hir return: guideline still has a name.