Uploading a technique with an empty tactic ("") should either ignore the value or reject the layer

[isaisabel]

As a user, I want the Navigator to catch improperly formatted layers.

Currently, uploading a layer with the following technique:

{
    "techniqueID": "T1001",
    "color": "#66ffaa",
    "tactic": "",
    "comment": "example comment",
    "enabled": true,
    "metadata": [],
    "showSubtechniques": false
}

Will not show the annotation because the tactic is matched to "". Since there are no tactics with an empty shortname this will essentially hide the annotation.

The Navigator should either:

• Treat "" tactic values as if they are undefined and map the annotation across tactics, or:
• Treat an empty string as an invalid value, and reject the layer from the upload

The current result, while adhering to the layer schema, is very hard to debug since it's easy to assume that an empty tactic string is the same as omitting the tactic field altogether.