fix: accept null for address parameter in FetchQuoteParams

Update `FetchQuoteParams` to accept `string | null` for the `address` field
to match the return type of `activeAddress` from `@txnlab/use-wallet-*`.
This eliminates the need for `|| undefined` fallbacks in consuming code.

The client normalizes null to undefined internally when creating quotes.

Author: drichar

examples/react/src/App.tsx

@@ -46,7 +46,7 @@ function App() {
         fromASAID: fromAsset,
         toASAID: toAsset,
         amount: amountInBaseUnits,
-        address: activeAddress || undefined,
+        address: activeAddress,
       })
       console.log('Quote received', quoteResult)
 

packages/deflex/src/client.ts

@@ -301,7 +301,7 @@ export class DeflexClient {
       ...response,
       quote: response.quote === '' ? 0n : BigInt(response.quote),
       amount: BigInt(params.amount),
-      address: params.address,
+      address: params.address ?? undefined,
       createdAt: Date.now(),
     }
   }

packages/deflex/src/types.ts

@@ -73,7 +73,7 @@ export interface FetchQuoteParams {
   readonly optIn?: boolean
 
   /** Address of the account that will perform the swap (required if autoOptIn is enabled) */
-  readonly address?: string
+  readonly address?: string | null
 }
 
 /**