Fix an exception when no capabilities are read

The intended behavior for Sink.get_source_cap() when no
Source_Capabilities are read was to return an empty list.  This wasn't
done before, instead raising a ValueError when read_pdo() tried to parse
"No Source_Capabilities" as an UnknownPDO.  Now we're a bit more
cautious, and the bug is fixed.

pdbuddy/__init__.py

@@ -427,6 +427,8 @@ def read_pdo(text):
                 peak_i=peak_i,
                 v=v,
                 i=i)
+    elif pdo_type == "No Source_Capabilities":
+        return None
     else:
         # Make an UnknownPDO
         return UnknownPDO(value=int(pdo_type, 16))
@@ -445,6 +447,8 @@ def read_pdo_list(text):
     # Read the PDOs
     pdo_list = []
     for start, end in zip(pdo_start_list[:-1], pdo_start_list[1:]):
-        pdo_list.append(read_pdo(text[start:end]))
+        pdo = read_pdo(text[start:end])
+        if pdo is not None:
+            pdo_list.append(pdo)
 
     return pdo_list

setup.py

@@ -5,7 +5,7 @@ with open("README.rst") as file:
 
 setup(
     name = "pd-buddy-python",
-    version = "0.3.0",
+    version = "0.3.1",
     author = "Clayton G. Hobbs",
     author_email = "clay@lakeserv.net",
     description = ("Python library for configuring PD Buddy Sink devices"),

test_pdbuddy/__init__.py

@@ -133,6 +133,9 @@ class SinkTestCase(unittest.TestCase):
         except ValueError:
             self.skipTest("Unknown value returned by PD Buddy Sink")
 
+    def test_get_source_cap(self):
+        self.assertIsInstance(self.pdbs.get_source_cap(), list)
+
     def test_send_command_invalid(self):
         with self.assertRaises(KeyError):
             self.pdbs.send_command("foo bar")