There is no single, specific god for that role, because which gods are honored varies by nation, and sometimes even municipality.
For example, in the late DR 1470s, Bane was the principal deity around the Moonsea - his high temple was in Mulmaster (southeastern Moonsea), he had a substantial temple in Phlan (northwestern Moonsea), and was popular among the Zhentarim. For a time even, Bane was the only officially permitted deity in Thay. He was a lawful evil god, with tyranny in his portfolio. A tyrant would not allow somebody else to determine laws in his realm, let alone enforce them.
In other areas, the situation is different. Any of the lawful deities could be called upon by those practicing law, or no deity at all. In the Realms, deities are very much driven by the power of the people - there are multiple points in the history of Faerun were gods have died because people stopped believing in them. While undoubtedly powerful, gods need the people more than people need gods. There is no requirement for a deity's rules to be involved in law enforcement or trials.