[dv/chip] Fix chip level jtag related warnings

### Description

Warning-[CNST-PPRW] Constraint randomize NULL object warning
../src/lowrisc_dv_dv_lib_0/dv_base_test.sv, 41
Member object 'm_jtag_agent_cfg' was found to be NULL during randomization.
Please make sure all random variables/arrays/function calls being randomized
are allocated fully and properly.

Warning-[CNST-PPRW] Constraint randomize NULL object warning
../src/lowrisc_dv_dv_lib_0/dv_base_test.sv, 41
Member object 'jtag_dmi_ral' was found to be NULL during randomization.
Please make sure all random variables/arrays/function calls being randomized
are allocated fully and properly.

Start at ../src/lowrisc_dv_dv_lib_0/dv_base_test.sv line 41 and reproduce the chip-level DV run that emits the warnings. Trace how m_jtag_agent_cfg and jtag_dmi_ral are initialized before randomization; done means the NULL-object warnings no longer appear and the relevant run passes.

Written by the indexing model from the issue text.