Fixes #278 - No longer warn of missing initializer with -Wextra

Added union initializer to static 'empty_node'. It is being
initialized to NULL (perhaps this should be initialized to the empty
string? it doesn't really matter, though.)
Merge pull request #278 from caitp/issue-278

[warning] src/libwebvtt/node.c:36: warning: missing initializer (-Wall -Wextra)