DragValue not support empty content in text editing mode

[tkkcc]

A normal way to edit 2 into 0 is backspace + 0. Current DragValue does not support this.

[xTibor]

This is a regression introduced somewhere between 0.19.0 and master. There are also other issues editing DragValues on master:

• Cannot enter decimal points after numbers anymore. For example you cannot turn "0" into "0.1" because DragValue swallows those decimal points.

• Entering negative signs in front of numbers are broken. You cannot turn "0" into "-0.1" because DragValue swallows negative signs in front of zeroes. It started eagerly and incorrectly normalizing negative zero into zero while in edit mode.

[xTibor]

PR #2342 has caused this regression.