main: Don’t print translation header with `--help`

The correct way to tell `g_option_context_new()` not to print any
information after the options is to pass `NULL`. Passing the empty
string results in a call to `gettext ("")`, which returns the
translation’s header.

This fixes the output of `accounts-daemon --help`.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
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build-fedora

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