aconfig: cache: reject empty namespace and name fields

Add invariant to struct Cache: all flag namespace and name fields added
to the cache are required to be non-empty strings.

Bug: 279485059
Test: atest aconfig.test
Change-Id: I5ff34ec8feccc19e52241d4221fc87699518f3ff
diff --git a/tools/aconfig/src/commands.rs b/tools/aconfig/src/commands.rs
index fcd8e9d..324f7d5 100644
--- a/tools/aconfig/src/commands.rs
+++ b/tools/aconfig/src/commands.rs
@@ -58,7 +58,7 @@
     declarations: Vec<Input>,
     values: Vec<Input>,
 ) -> Result<Cache> {
-    let mut cache = Cache::new(namespace.to_owned());
+    let mut cache = Cache::new(namespace.to_owned())?;
 
     for mut input in declarations {
         let mut contents = String::new();