Update charm libraries

lib/charms/loki_k8s/v0/loki_push_api.py

@@ -480,7 +480,7 @@ def _alert_rules_error(self, event):
 
 # Increment this PATCH version before using `charmcraft publish-lib` or reset
 # to 0 if you are raising the major API version
-LIBPATCH = 26
+LIBPATCH = 28
 
 logger = logging.getLogger(__name__)
 
@@ -2116,15 +2116,18 @@ def _download_and_push_promtail_to_workload(self, promtail_info: dict) -> None:
                - "binsha": sha256 sum of unpacked promtail binary
         """
         # Check for Juju proxy variables and fall back to standard ones if not set
-        proxies: Optional[Dict[str, str]] = {}
-        if proxies and os.environ.get("JUJU_CHARM_HTTP_PROXY"):
-            proxies.update({"http": os.environ["JUJU_CHARM_HTTP_PROXY"]})
-        if proxies and os.environ.get("JUJU_CHARM_HTTPS_PROXY"):
-            proxies.update({"https": os.environ["JUJU_CHARM_HTTPS_PROXY"]})
-        if proxies and os.environ.get("JUJU_CHARM_NO_PROXY"):
-            proxies.update({"no_proxy": os.environ["JUJU_CHARM_NO_PROXY"]})
-        else:
-            proxies = None
+        # If no Juju proxy variable was set, we set proxies to None to let the ProxyHandler get
+        # the proxy env variables from the environment
+        proxies = {
+            # The ProxyHandler uses only the protocol names as keys
+            # https://docs.python.org/3/library/urllib.request.html#urllib.request.ProxyHandler
+            "https": os.environ.get("JUJU_CHARM_HTTPS_PROXY", ""),
+            "http": os.environ.get("JUJU_CHARM_HTTP_PROXY", ""),
+            # The ProxyHandler uses `no` for the no_proxy key
+            # https://github.com/python/cpython/blob/3.12/Lib/urllib/request.py#L2553
+            "no": os.environ.get("JUJU_CHARM_NO_PROXY", ""),
+        }
+        proxies = {k: v for k, v in proxies.items() if v != ""} or None
 
         proxy_handler = request.ProxyHandler(proxies)
         opener = request.build_opener(proxy_handler)