Fix PGPKey.from_blob('') not raising an exception. Fixes #199.

tests/test_99_regressions.py

@@ -332,3 +332,9 @@ def test_oneline_cleartext(sf, cleartext):
     assert dearmor['magic'] == 'SIGNATURE'
     # No newline at the end
     assert dearmor['cleartext'] == cleartext
+
+
+@pytest.mark.regression(issue=199)
+def test_armorable_empty_str():
+    with pytest.raises(ValueError, message='Expected: ASCII-armored PGP data'):
+        Armorable.ascii_unarmor('')