[FIX] web: traceback on line graphs with only one data

Graph tooltip destruction was fixed with commit https://github.com/odoo/odoo/commit/86252428846607cd5f4b18cef8de49a8cd0b151a#diff-5de06eee7a2eeae066aa1348a1528fe9L100.
Unfortunately, the commit supposed that the 'display_' methods always
returned a graph, which is wrong.

opw-773759