Handle empty messages in qt_message_handler

I can't reproduce this, but someone on KDE reported always getting a crash (as
msg.splitlines()[0] gives an IndexError) when trying to select a file with
Qt 5.9.3.

qutebrowser/utils/log.py

@@ -422,6 +422,9 @@ def qt_message_handler(msg_type, context, msg):
                 'with: -9805',  # flake8: disable=E131
         ]
 
+    if not msg:
+        msg = "Logged empty message!"
+
     if any(msg.strip().startswith(pattern) for pattern in suppressed_msgs):
         level = logging.DEBUG
     else:

tests/unit/utils/test_log.py

@@ -25,12 +25,15 @@ import itertools
 import sys
 import warnings
 
+import attr
 import pytest
 import pytest_catchlog
 
 from qutebrowser.utils import log
 from qutebrowser.misc import utilcmds
 
+from PyQt5 import QtCore
+
 
 @pytest.fixture(autouse=True)
 def restore_loggers():
@@ -252,3 +255,22 @@ def test_ignore_py_warnings(caplog):
     assert len(caplog.records) == 1
     msg = caplog.records[0].message.splitlines()[0]
     assert msg.endswith("UserWarning: not hidden")
+
+
+class TestQtMessageHandler:
+
+    @attr.s
+    class Context:
+
+        """Fake QMessageLogContext."""
+
+        function = attr.ib(default=None)
+        category = attr.ib(default=None)
+        file = attr.ib(default=None)
+        line = attr.ib(default=None)
+
+    def test_empty_message(self, caplog):
+        """Make sure there's no crash with an empty message."""
+        log.qt_message_handler(QtCore.QtDebugMsg, self.Context(), "")
+        assert len(caplog.records) == 1
+        assert caplog.records[0].msg == "Logged empty message!"