wxGUI/g.gui.mapswipe: fix hit Apply button if first and second raster aren't chosen

[tmszi]

To Reproduce:

1. Launch Map Swipe window g.gui.mapswipe
2. On the Select raster maps dialog (don't choose any first and second raster map)
3. Hit Apply button

Error message:

Traceback (most recent call last):
  File "/usr/lib64/grass79/gui/wxpython/mapswipe/dialogs.py", line 69, in <lambda>
    self.btnApply.Bind(wx.EVT_BUTTON, lambda evt: self._apply())
  File "/usr/lib64/grass79/gui/wxpython/mapswipe/dialogs.py", line 231, in _apply
    self.applyChanges.emit()
  File "/usr/lib64/grass79/etc/python/grass/pydispatch/signal.py", line 230, in emit
    dispatcher.send(signal=self, *args, **kwargs)
  File "/usr/lib64/grass79/etc/python/grass/pydispatch/dispatcher.py", line 350, in send
    **named
  File "/usr/lib64/grass79/etc/python/grass/pydispatch/robustapply.py", line 60, in robustApply
    return receiver(*arguments, **named)
  File "/usr/lib64/grass79/gui/wxpython/mapswipe/frame.py", line 399, in OnApplyInputChanges
    res1 = self.SetFirstRaster(name=self.rasters['first'])
  File "/usr/lib64/grass79/gui/wxpython/mapswipe/frame.py", line 424, in SetFirstRaster
    if raster['fullname']:
KeyError: 'fullname'

Show error message dialog if you don't choose first and second raster map:

[petrasovaa]

When you enter invalid raster name in one field and leave the other one empty, you won't get any error, so I think this case should be fixed.

[petrasovaa]

I further suggest these changes to cover cases when one raster is empty and the other is invalid:

+++ b/gui/wxpython/mapswipe/frame.py
@@ -398,14 +398,15 @@ class SwipeMapFrame(DoubleMapFrame):
             self.rasters['first'], self.rasters['second'] = first, second
             res1 = self.SetFirstRaster(name=self.rasters['first'])
             res2 = self.SetSecondRaster(name=self.rasters['second'])
-            if not (res1 and res2) and first and second:
+            if not (res1 and res2) and (first or second):
                 message = ''
-                if not res1:
+                if first and not res1:
                     message += _("Map <%s> not found. ") % self.rasters[
                         'first']
-                if not res2:
+                if second and not res2:
                     message += _("Map <%s> not found.") % self.rasters[
                         'second']
+                if message:
                     GError(parent=self, message=message)
                     return
             self.ZoomToMap()