Permit access to all mirrors if the list of allowed mirrors is empty

The list of allowed mirrors is empty by default, making apt-cacher-rs
unusable in the default configuration. Therefore, this commit changes
the behavior to allow access to all mirrors if the list is empty.

Author: madmajestro

Diff for: apt-cacher-rs.conf

@@ -49,6 +49,7 @@
 #aliases = []
 
 # List of allowed mirrors.
+# If no mirrors are listed no restrictions are applied.
 # Example:
 #   allowed_mirrors = [ 'deb.debian.org' ]
 #allowed_mirrors = []

Diff for: src/main.rs

@@ -1490,6 +1490,10 @@ enum CacheFileStat<'a> {
 }
 
 fn is_host_allowed(requested_host: &str) -> bool {
+    if global_config().allowed_mirrors.is_empty() {
+        return true;
+    }
+
     global_config()
         .allowed_mirrors
         .iter()