unify default value for null cell in vtk output

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@58155 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.out.vtk/parameters.c

@@ -45,7 +45,7 @@ void set_params()
     param.null_val->type = TYPE_DOUBLE;
     param.null_val->required = NO;
     param.null_val->description = _("Value to represent no data cell");
-    param.null_val->answer = "-10.0";
+    param.null_val->answer = "-99999.99";
 
     param.elev = G_define_option();
     param.elev->key = "elevation2d";

temporal/t.rast.out.vtk/t.rast.out.vtk.py

@@ -46,7 +46,7 @@
 #% description: Value to represent no data cell
 #% required: no
 #% multiple: no
-#% answer: -10.0
+#% answer: -99999.99
 #%end
 
 #%flag