[folks] core: Add missing initialisation of NoteDetails member

[Philip Withnall <pwithnall src gnome org>]

commit 0b574c5860457273ff61d0f1e70641ae643d9676
Author: Philip Withnall <withnall endlessm com>
Date:   Sun Jul 2 20:49:55 2017 +0100

    core: Add missing initialisation of NoteDetails member
    
    Otherwise it’s implicitly initialised to null, which causes assertion
    failures in the tests later on (it’s a state the rest of the code in
    NoteDetails does not expect).
    
    Signed-off-by: Philip Withnall <withnall endlessm com>

 folks/note-details.vala |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/folks/note-details.vala b/folks/note-details.vala
index 5b7df9c..37521cd 100644
--- a/folks/note-details.vala
+++ b/folks/note-details.vala
@@ -33,7 +33,7 @@ using GLib;
  */
 public class Folks.NoteFieldDetails : AbstractFieldDetails<string>
 {
-  private string _id;
+  private string _id = "";
   /**
    * {@inheritDoc}
    */