fix: encode strings (even empty strings) when stored

This fixes a bug where `CallContext.store('')` would throw an error.

src/call-context.ts

@@ -451,14 +451,14 @@ export class CallContext {
 
   /** @hidden */
   [STORE](input?: string | Uint8Array): number | null {
-    if (!input) {
-      return null;
-    }
-
     if (typeof input === 'string') {
       input = this.#encoder.encode(input);
     }
 
+    if (!input) {
+      return null;
+    }
+
     if (input instanceof Uint8Array) {
       if (input.buffer.constructor === this.#arrayBufferType) {
         // no action necessary, wrap it up in a block