Holdings editor regression: Can no longer remove a default item alert type

[Jane Sandberg]

Affects today's main and rel_3_13 branches. rel_3_12 is unaffected.

Steps to recreate:
1. Open your favorite bib record.
2. Press the Add Holdings button
3. On the Preferences tab, under Item Attribute Settings, choose a Default Alert Type
4. Move to another tab, then move back to Preferences. We get the expected behavior: that the Default Alert Type is still selected
5. Use backspace to remove the Default Alert Type.
6. Again, move to another tab, then move back to Preferences. We get unexpected behavior: that the Default Alert Type is still selected, even though we totally deleted it in the previous step.

Alternatively, run from the eg2 folder: `npx nightwatch nightwatch/src/staff/cat/holdings-editor-config.spec.ts` to have a robot do the above steps.

I suspect this came in with 3.13 combobox changes, so it's possible that other interfaces are affected in similar ways.

Previously (and in today's rel_3_12 branch), you could clear your Default Alert Type this way. Now, the only way for a user to clear it in the UI is to go into their workstation settings and delete *all* of their holdings editor preferences in the Server Workstation Prefs tab -- far from ideal.

[Dan Briem (dbriem)]

The isEditable Input() on the combobox defaults to false, which sets the editable property on the ngbTypeahead. I think this means the ngModel binding is limited to entry selections, so clearing the combobox would cause the model value to become undefined instead of an empty string.

It looks like the logic to clear the combobox on blur depends on the model value being an empty string.

Setting the isEditable property to true on the Default Alert Type combobox might solve the problem here, but I'm not sure which other combobox contexts depend on the default behavior.