Database.ReadThroughputAsync throw CosmosException if no database throughput

[donghexu]

We are continuously addressing and improving the SDK, if possible, make sure the problem persist in the latest SDK version.

Describe the bug
Database.ReadThroughputAsync() throws CosmosException in 3.1.1, if there is no database throughput. In 3.0.0, it returns Nullable as expected.

To Reproduce
Create a database without throughput, and use 3.1.1 to read the throughput, it will throw exception.

Expected behavior
Should be consistent with 3.0.0 behavior, which returns nullable.

Actual behavior
Throw CosmosException

Environment summary
SDK Version: 3.1.1
OS Version (e.g. Windows, Linux, MacOSX) Windows

[j82w]

@simplynaveen20 is this a regression?