ZSQLCatalog: Tolerate non-existent tables in sql_search_tables.

Such tables are not visible in the ZMI (which is arguably a bug on its
own, although the ZMI is deprecated), but even accessing the setting
tab is broken as it runs these methods.
This change at least allows accessing stuff to start fixing.

product/ZSQLCatalog/SQLCatalog.py

@@ -1033,7 +1033,7 @@ class Catalog(Folder,
     add_key = keys.add
     table_dict = self._getCatalogSchema()
     for table in self.getCatalogSearchTableIds():
-      for field in table_dict[table]:
+      for field in table_dict.get(table, ()):
         add_key(field)
         add_key('%s.%s' % (table, field))  # Is this inconsistent ?
     for related in self.getSQLCatalogRelatedKeyList():
@@ -1054,7 +1054,7 @@ class Catalog(Folder,
     result = {}
     table_dict = self._getCatalogSchema()
     for table in self.getCatalogSearchTableIds():
-      for field in table_dict[table]:
+      for field in table_dict.get(table, ()):
         result.setdefault(field, []).append(table)
         result.setdefault('%s.%s' % (table, field), []).append(table) # Is this inconsistent ?
     return result
@@ -1069,7 +1069,7 @@ class Catalog(Folder,
     keys = set()
     table_dict = self._getCatalogSchema()
     for table in self.getCatalogSearchTableIds():
-      for field in table_dict[table]:
+      for field in table_dict.get(table, ()):
         keys.add('%s.%s' % (table, field))
     return sorted(keys)