wxGUI/dbmgr: fix get statistics for character column type if it contains NULL value

[tmszi]

To Reproduce
Steps to reproduce the behavior:

1. Launch wxGUI Attribute Table Manager g.gui.dbmgr map=geology (Basic NC location)
2. Erase (set NULL) column 'GEO_NAME' value for row with column cat = 1
3. Right mouse click on the 'GEO_NAME' column and choose from the menu Statistics item
4. An empty Statistics window opened, and you see error message in the wxGUI Console tab

Error message

Traceback (most recent call last):
  File "/usr/lib64/grass79/gui/wxpython/dbmgr/base.py", line
654, in OnFieldStatistics

column=self.GetColumn(self._col).GetText(),
  File "/usr/lib64/grass79/gui/wxpython/dbmgr/base.py", line
4054, in Update

varSum += float(var)
ValueError
:
could not convert string to float:

[petrasovaa]

Looks good, thanks!