Let triage_tests support test modules with only figure_equals tests.

If there's only check_figure_equals tests in a module (e.g. test_pickle,
test_axis) then there's no directory in the baseline_images test, but
it still makes sense to run triage_tests in case of test failure.
Previously, the missing directory caused an error; this patches fixes
that.  To test this, change e.g. one of the check_figures_equal
tests in test_pickle to make it fail, run that test, and run
tools/triage_tests.py.

Author: anntzer

tools/triage_tests.py

@@ -263,7 +263,7 @@ def __init__(self, path, root, source):
             ]
         self.thumbnails = [self.dir / x for x in self.thumbnails]
 
-        if not Path(self.destdir, self.generated).exists():
+        if self.destdir is None or not Path(self.destdir, self.generated).exists():
             # This case arises from a check_figures_equal test.
             self.status = 'autogen'
         elif ((self.dir / self.generated).read_bytes()
@@ -281,7 +281,6 @@ def get_dest_dir(self, reldir):
             path = self.source / baseline_dir / reldir
             if path.is_dir():
                 return path
-        raise ValueError(f"Can't find baseline dir for {reldir}")
 
     @property
     def display(self):