Raise error when accessing attributes of empty ATG

When accessing attributes like xmin of an empty ATG, the exception
raised should inform the user that they are trying to do something
meaningless rather than a cryptic numpy error.

Adds testing to this effect.

src/aligned_textgrid/aligned_textgrid.py

@@ -41,6 +41,8 @@ class for each tier within each tier group. Say, if only the first speaker
             The entry classes for each tier within a tier group.
         tier_groups (list[TierGroup] | list[]):
             A list of `TierGroup` or an empty list.
+        tier_names (list[str]):
+            A list of names for tiers in tier_groups.
         xmax (float):
             Maximum time
         xmin (float):
@@ -178,8 +180,6 @@ def _init_empty(self):
         self.contains = self.tier_groups
         self.entry_classes = []
         self.tg_tiers = None
-        self.xmax = 0.0
-        self.xmin = 0.0
 
     def _nestify_tiers(
         self,
@@ -276,14 +276,20 @@ def _relate_tiers(self):
 
     @property
     def tier_names(self):
+        if len(self) == 0:
+            raise ValueError('No tier names in an empty TextGrid.')
         return [x.tier_names for x in self.tier_groups]
 
     @property
     def xmin(self):
+        if len(self) == 0:
+            raise ValueError('No minimum time for empty TextGrid.')
         return np.array([tgroup.xmin for tgroup in self.tier_groups]).min()
 
     @property
     def xmax(self):
+        if len(self) == 0:
+            raise ValueError('No maximum time for empty TextGrid.')
         return np.array([tgroup.xmax for tgroup in self.tier_groups]).max()
 
     def interleave_class(

tests/test_aligned_textgrid.py

@@ -138,8 +138,10 @@ def test_get_class_by_name(self):
 
         missing_class1 = self.atg1.get_class_by_name("Foo")
         missing_class2 = self.atg2.get_class_by_name("Foo")
+        missing_class4 = self.atg4.get_class_by_name("Foo")
         assert missing_class1 is None
         assert missing_class2 is None
+        assert missing_class4 is None
 
         target_classes = self.atg3.get_class_by_name("MyWord")
         assert len(target_classes) > 1
@@ -158,6 +160,15 @@ def test_empty_class_indexing(self):
             empty_idx_list = []
             self.atg4[empty_idx_list]
 
+    def test_empty_class_attributes(self):
+        with pytest.raises(ValueError):
+            self.atg4.xmin
+
+        with pytest.raises(ValueError):
+            self.atg4.xmax
+
+        with pytest.raises(ValueError):
+            self.atg4.tier_names
 
 class TestInGetLen:
     atg = AlignedTextGrid(