SEP-10: The transaction's ManageData operation value should not be null

[overcat]

Describe the bug
In SEP-10, when we check the value of ManagaData, we should first check whether the value exists, otherwise, we will get a TypeError.

js-stellar-sdk/src/utils.ts

Line 183 in e7dc9b1

// verify base64

What version are you on?
v6.2.0

To Reproduce
See PR's test case. #591

Expected behavior
The SDK normally handles cases where the value of ManageData is null.

I think other SDKs also have this problem.