fix(graphql): make USDT wallet balance nullable

dev/apollo-federation/supergraph.graphql

@@ -2039,7 +2039,7 @@ type UsdtWallet implements Wallet
   @join__type(graph: PUBLIC)
 {
   accountId: ID!
-  balance: FractionalCentAmount!
+  balance: FractionalCentAmount
   id: ID!
   isExternal: Boolean!
   lnurlp: Lnurl

src/graphql/admin/schema.graphql

@@ -572,7 +572,7 @@ A wallet belonging to an account which contains a USDT balance and a list of tra
 """
 type UsdtWallet implements Wallet {
   accountId: ID!
-  balance: FractionalCentAmount!
+  balance: FractionalCentAmount
   id: ID!
   isExternal: Boolean!
   lnurlp: Lnurl

src/graphql/public/schema.graphql

@@ -1654,7 +1654,7 @@ A wallet belonging to an account which contains a USDT balance and a list of tra
 """
 type UsdtWallet implements Wallet {
   accountId: ID!
-  balance: FractionalCentAmount!
+  balance: FractionalCentAmount
   id: ID!
   isExternal: Boolean!
   lnurlp: Lnurl

src/graphql/shared/types/object/usdt-wallet.ts

@@ -56,7 +56,7 @@ const UsdtWallet = GT.Object<Wallet>({
     },
 
     balance: {
-      type: GT.NonNull(FractionalCentAmount),
+      type: FractionalCentAmount,
       resolve: async (source) => {
         const balance = await Wallets.getBalanceForWallet({
           walletId: source.id,

test/flash/unit/graphql/wallet-balance-validation.spec.ts

@@ -0,0 +1,30 @@
+import { parse, validate } from "graphql"
+
+import { gqlMainSchema } from "@graphql/public"
+
+describe("wallet balance query validation", () => {
+  it("allows querying USD and USDT wallet balances with the same response name", () => {
+    const query = parse(`
+      query Me {
+        me {
+          defaultAccount {
+            wallets {
+              ... on UsdtWallet {
+                id
+                balance
+              }
+              ... on UsdWallet {
+                id
+                balance
+              }
+            }
+          }
+        }
+      }
+    `)
+
+    const errors = validate(gqlMainSchema, query)
+
+    expect(errors).toEqual([])
+  })
+})