Fix a bug where Synapse fails to start if a signing key file contains…

… an empty line. (#13738)

changelog.d/13738.bugfix

@@ -0,0 +1 @@
+Fix a bug where Synapse fails to start if a signing key file contains an empty line.

synapse/config/key.py

@@ -217,7 +217,18 @@ def read_signing_keys(self, signing_key_path: str, name: str) -> List[SigningKey
 
         signing_keys = self.read_file(signing_key_path, name)
         try:
-            return read_signing_keys(signing_keys.splitlines(True))
+            loaded_signing_keys = read_signing_keys(
+                [
+                    signing_key_line
+                    for signing_key_line in signing_keys.splitlines(keepends=False)
+                    if signing_key_line.strip()
+                ]
+            )
+
+            if not loaded_signing_keys:
+                raise ConfigError(f"No signing keys in file {signing_key_path}")
+
+            return loaded_signing_keys
         except Exception as e:
             raise ConfigError("Error reading %s: %s" % (name, str(e)))