Fixed bug in PhotosDB.__repr__ and added test for empty database

tests/Empty-Library-4.0-3461.7.150.photoslibrary/private/com.apple.photoanalysisd/GraphService/PhotoAnalysisServicePreferences.plist

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<plist version="1.0">
+<dict>
+	<key>PhotoAnalysisGraphLastBackgroundGraphRebuildJobDate</key>
+	<date>2019-12-27T23:19:59Z</date>
+</dict>
+</plist>