fuzzy_url: Raise InvalidUrlError on empty URLs.

Before we raised QtValueError (via qtutils.ensure_valid), but maybe there are
more callers out there which call fuzzy_url with an empty input - and it makes
more sense to raise InvalidUrlError which gets displayed to the user than
raising QtValueError which is more like an assertion.

CHANGELOG.asciidoc

@@ -43,6 +43,7 @@ Fixed
   the tab it belongs to.
 - Fixed crash when downloading a file without any path information (e.g a
   magnet link).
+- Fixed crashes when opening an empty URL (e.g. via pasting).
 
 v0.5.1
 ------

qutebrowser/utils/urlutils.py

@@ -172,6 +172,7 @@ def fuzzy_url(urlstr, cwd=None, relative=False, do_search=True):
     """
     urlstr = urlstr.strip()
     expanded = os.path.expanduser(urlstr)
+
     if os.path.isabs(expanded):
         path = expanded
     elif relative and cwd:
@@ -199,7 +200,7 @@ def fuzzy_url(urlstr, cwd=None, relative=False, do_search=True):
             url = qurl_from_user_input(urlstr)
     log.url.debug("Converting fuzzy term {} to URL -> {}".format(
                   urlstr, url.toDisplayString()))
-    if do_search and config.get('general', 'auto-search'):
+    if do_search and config.get('general', 'auto-search') and urlstr:
         qtutils.ensure_valid(url)
     else:
         if not url.isValid():

tests/unit/utils/test_urlutils.py

@@ -233,6 +233,11 @@ class TestFuzzyUrl:
         with pytest.raises(exception):
             urlutils.fuzzy_url('foo', do_search=do_search)
 
+    @pytest.mark.parametrize('url', ['', ' '])
+    def test_empty(self, url):
+        with pytest.raises(urlutils.InvalidUrlError):
+            urlutils.fuzzy_url(url, do_search=True)
+
 
 @pytest.mark.parametrize('url, special', [
     ('file:///tmp/foo', True),