Is CTS necessary on a AES-CBC DRBG?
Normally you don't want to reverse the encryption used within a DRBG. Schemes like PKCS#7 padding and CTS are required to deterministically reverse the padding during decryption. If you just want to have a final block, you should be fine by using zero padding until the end of the block.

Of course, if this zero padding is actually harming the output of your DRBG depends on the algorithm.