Fix privSz for wp_dh_get_params

[aidangarske]

Description

• wp_dh_get_params was returning the pub key size when it should return the private key size here.
• Added a test case for NULL data entries for size queries. Before we where not actually hitting this line in the unit test.
• Private key size queries with NULL where actually failing as well like this:

#### Start: 57 - test_dh_get_params
MSG: EVP_PKEY_get_params failed for param priv in mode Null data (WolfProvider (0))
#### FAILED: 57 - test_dh_get_params (err: 1)

[padelsbach]

Good catch!

[padelsbach]

Is there a test case we should add for this?

[padelsbach]

Nice job