Safety Valve Override

- Support 'allow.empty.filter', defaulting to 'yes', but if 'no' disallows the
  combination of a write command and an empty filter.  Ordinarily this is just
  a warning that requires confirmation (thanks to Lee Lieske).

src/Config.cpp

@@ -81,6 +81,7 @@ std::string Config::_defaults =
   "#                                              # Comma-separated list.  May contain any subset of:\n"
   "#verbose=blank,header,footnote,label,new-id,new-uuid,affected,edit,special,project,sync,filter\n"
   "confirmation=yes                               # Confirmation on delete, big changes\n"
+  "allow.empty.filter=yes                         # An empty filter gets a warning and requires confirmation\n"
   "indent.annotation=2                            # Indent spaces for annotations\n"
   "indent.report=0                                # Indent spaces for whole report\n"
   "row.padding=0                                  # Left and right padding for each row of report\n"