Aborting a grid cell edit action on an empty cell results in a Debug assertion

[jeroens]

To reproduce:

• In the scientific interface load any model
• In the basic inputs tab, double-click an editable cell without a value
• Now click another cell

The GUI sends the empty value for the cell as a new value, which chokes the core

[joeb]

ICoreInputOutput.setVariable(...) if the variable being set is not GroupName? then empty string is turned into zero. This assumes that the user want to clear the existing value.

[jeroens]

Done