Issue 2212. Improve NFToken.URI handling for empty and undefined values (#2218)

* Handle undefined and null values in transactions better.

packages/xrpl/test/models/NFTokenMint.test.ts

@@ -68,6 +68,26 @@ describe('NFTokenMint', function () {
     )
   })
 
+  it(`throws w/ URI being an empty string`, function () {
+    const invalid = {
+      TransactionType: 'NFTokenMint',
+      Account: 'rWYkbWkCeg8dP6rXALnjgZSjjLyih5NXm',
+      Fee: '5000000',
+      Sequence: 2470665,
+      Flags: NFTokenMintFlags.tfTransferable,
+      NFTokenTaxon: 0,
+      Issuer: 'r9LqNeG6qHxjeUocjvVki2XR35weJ9mZgQ',
+      TransferFee: 1,
+      URI: '',
+    } as any
+
+    assert.throws(
+      () => validate(invalid),
+      ValidationError,
+      'NFTokenMint: URI must not be empty string',
+    )
+  })
+
   it(`throws w/ URI not in hex format`, function () {
     const invalid = {
       TransactionType: 'NFTokenMint',