Fix error in Mercurial when no offset is specified

Summary:
When viewing the history of a Mercurial diffusion repository the server return a 500. This is
because a function call to array_slice requires the offset to be set to an integer. So when it is
not specified it can be set to 0 as a default.

The error is as follows:

ERROR 8192: array_slice(): Passing null to parameter #2 ($offset) of type int is deprecated at [/usr/local/www/phorge/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php:167]; PHP message: arcanist(head=master, ref.master=3cb117684f4e), phorge(head=master, ref.master=4bf5c452eb28); PHP message:   #0 array_slice(array, NULL) called at [<phorge>/src/applications/diffusion/conduit/DiffusionHistoryQueryConduitAPIMethod.php:167]

Test Plan: The history tab should not return a 500 when not specifying an offset

Reviewers: O1 Blessed Committers, valerio.bozzolan, speck

Reviewed By: O1 Blessed Committers, valerio.bozzolan, speck

Subscribers: aklapper, speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25673