Fix optional default initialization
Fix optional default initialization

Summary: Initialize Optional internal memory, so that "un-initialized memory" tools don't flag its callsites.

Reviewed By: yfeldblum

Differential Revision: D3960462

fbshipit-source-id: 3bd0109959fb93e040fa2e874f586b3508e46dd2

rklabs/folly,floxard/folly,rklabs/folly,Orvid/folly,facebook/folly,facebook/folly,rklabs/folly,facebook/folly,Orvid/folly,floxard/folly,rklabs/folly,facebook/folly,charsyam/folly,Orvid/folly,Orvid/folly,charsyam/folly,Orvid/folly,facebook/folly,charsyam/folly,floxard/folly,rklabs/folly,charsyam/folly,floxard/folly,floxard/folly