Allow for empty DownloadArchive tags

This also updates the type hints for the PackageUpdate class, to make
it easier to understand the datatypes. It also adds some assertions
to prove what types they are.

aqt/archives.py

@@ -124,13 +124,21 @@ class PackageUpdate:
     description: str
     release_date: str
     full_version: str
-    dependencies: List[str]
-    auto_dependon: Optional[List[str]]
-    downloadable_archives: Optional[List[str]]
+    dependencies: Iterable[str]
+    auto_dependon: Iterable[str]
+    downloadable_archives: Iterable[str]
     default: bool
     virtual: bool
     base: str
 
+    def __post_init__(self):
+        for iter_of_str in self.dependencies, self.auto_dependon, self.downloadable_archives:
+            assert isinstance(iter_of_str, Iterable) and not isinstance(iter_of_str, str)
+        for _str in self.name, self.display_name, self.description, self.release_date, self.full_version, self.base:
+            assert isinstance(_str, str)
+        for boolean in self.default, self.virtual:
+            assert isinstance(boolean, bool)
+
     @property
     def version(self):
         return Version.permissive(self.full_version)
@@ -219,7 +227,7 @@ class Updates:
     def merge(self, other):
         self.package_updates.extend(other.package_updates)
 
-    def get_depends(self, target: str):
+    def get_depends(self, target: str) -> Iterable[str]:
         # initialize
         filo = [target]
         packages = []
@@ -237,19 +245,19 @@ class Updates:
                                 filo.append(depend)
         return packages
 
-    def _get_text(self, item):
+    def _get_text(self, item) -> str:
         if item is not None and item.text is not None:
             return item.text
         else:
             return ""
 
-    def _get_list(self, item):
+    def _get_list(self, item) -> Iterable[str]:
         if item is not None and item.text is not None:
             return ssplit(item.text)
         else:
-            return None
+            return []
 
-    def _get_boolean(self, item):
+    def _get_boolean(self, item) -> bool:
         if "true" == item:
             return True
         else: