[polari] app: Fix setting debug flags with unset TP_DEBUG

[Florian Müllner <fmuellner src gnome org>]

commit a86208b00696c952c0b38b099512a57a38e753d8
Author: Florian Müllner <fmuellner gnome org>
Date:   Tue Jun 2 16:58:08 2020 +0200

    app: Fix setting debug flags with unset TP_DEBUG
    
    tp_debug_set_flags() misses a (nullable) annotation.
    Work around that by passing the empty string instead
    of null when the environment variable is unset.
    
    https://gitlab.gnome.org/GNOME/polari/-/issues/144

 src/application.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/application.js b/src/application.js
index 509c927c..762c6ded 100644
--- a/src/application.js
+++ b/src/application.js
@@ -32,7 +32,7 @@ var Application = GObject.registerClass({
         });
 
         GLib.set_prgname('polari');
-        Tp.debug_set_flags(GLib.getenv('TP_DEBUG'));
+        Tp.debug_set_flags(GLib.getenv('TP_DEBUG') || '');
         this._retryData = new Map();
         this._nickTrackData = new Map();
         this._demons = [];