Error loading historical proposal when metadata is an empty string

[wgwz]

If a proposal has an empty metadata string, then the proposal will not be loaded as a historical proposal in the groups page.

To reproduce:

• create a group with an address of yours as admin
• create a proposal for the group with EMPTY metadata, pass it and exec it
• observe that the historical proposal is not shown on the group page

[wgwz]

Closing because it's a duplicate of: #146