safe_join: raise ValueError on None parameters and update tests

Changed safe_join to raise ValueError if base or tail is None instead of returning 'None/path'.
Adjusted unit tests accordingly to expect exceptions for None inputs and kept empty-string handling valid.

Ref: https://chatgpt.com/share/68b55850-e854-800f-9702-09ea956b8dc4

filter_plugins/safe_join.py

@@ -1,19 +1,20 @@
 """
 Ansible filter plugin that joins a base string and a tail path safely.
-If the base is falsy (None, empty, etc.), returns an empty string.
+Raises ValueError if base or tail is None.
 """
 
 def safe_join(base, tail):
     """
     Safely join base and tail into a path or URL.
 
-    - base: the base string. If falsy, returns ''.
-    - tail: the string to append. Leading/trailing slashes are handled.
-    - On any exception, returns ''.
+    - base: the base string. Must not be None.
+    - tail: the string to append. Must not be None.
+    - On ValueError, caller should handle it.
     """
+    if base is None or tail is None:
+        raise ValueError("safe_join: base and tail must not be None")
+
     try:
-        if not base:
-            return ''
         base_str = str(base).rstrip('/')
         tail_str = str(tail).lstrip('/')
         return f"{base_str}/{tail_str}"