models: return None from MessageManager query methods
models: return None from MessageManager query methods

Otherwise, we get 500 errors on series views such as
https://patchew.org/XYZ/abc@def/ or message views such as
https://patchew.org/XYZ/abc@def/ghi@jkl/.

Signed-off-by: Paolo Bonzini <69d3ebcf5ee4e7f05a50dc23cd96655b730804b4@redhat.com>

patchew-project/patchew,patchew-project/patchew,patchew-project/patchew,patchew-project/patchew