do not abort on getting CSV out of uncompleted Table or Ranked Items fields

wcs/fields.py

@@ -971,6 +971,8 @@ class TableField(WidgetField):
         return '\n'.join([indent + x for x in r])
 
     def get_csv_heading(self):
+        if not self.columns:
+            return [self.label]
         labels = []
         for col in self.columns:
             for row in self.rows:
@@ -982,6 +984,8 @@ class TableField(WidgetField):
         return labels
 
     def get_csv_value(self, value):
+        if not self.columns:
+            return ['']
         values = []
         for i in range(len(self.columns)):
             for j in range(len(self.rows)):
@@ -1219,9 +1223,13 @@ class RankedItemsField(WidgetField):
         return indent + ' / '.join(values)
 
     def get_csv_heading(self):
+        if not self.items:
+            return [self.label]
         return [self.label] + ['']*(len(self.items)-1)
 
     def get_csv_value(self, value):
+        if not self.items:
+            return ['']
         if type(value) is not dict:
             value = {}
         items = value.items()