Make the filter list optional in the param module

Using the param module, if the list of filters is empty, no flux will be
computed and the module won't complain about '' missing filter.  Also,
switch the default value for this list of filters to the empty string,
because None means that there is no default value which makes the module
not usable without the list for filters parameters.

Closes #5

pcigale/creation_modules/param.py

@@ -48,7 +48,7 @@ class Param(CreationModule):
             "Filters for which the flux will be computed and added to the SED "
             "information dictionary. You can give several filter names "
             "separated by a & (don't use commas).",
-            None
+            ""
         ))
     ])
 
@@ -128,7 +128,8 @@ class Param(CreationModule):
 
         # Computation of fluxes
         filter_list = [item.strip() for item in
-                       self.parameters["filter_list"].split("&")]
+                       self.parameters["filter_list"].split("&")
+                       if item.strip() != '']
 
         for filter_ in filter_list:
             sed.add_info(