Safe guard: zero output allowed

[rajapars]

Allowing C_EncryptUpdate to return no data (cipherlen == 0)

[rajapars]

C_EncryptUpdate and C_DecryptUpdate may legitimately return zero-length output. The current library behavior re-sends the same function call in this case, which leads to incorrect results (e.g., input ABCD becomes AABBCCDD).

According to PKCS#11 specification section 5.2 (Conventions), a retry is only required when the function returns CKR_BUFFER_TOO_SMALL with a non-zero required output length.

Therefore, if the function returns CKR_OK with an output length of 0, the operation should not be re-invoked