Revert "fix(prisma-adapter): don't treat a null unique field as an update selector"

A null unique selector should keep failing instead of falling back to
updateMany, which can change several rows on providers that allow
multiple NULLs in a unique column.

packages/prisma-adapter/src/prisma-adapter.test.ts

@@ -300,35 +300,6 @@ describe("prisma-adapter", () => {
 		);
 	});
 
-	it("falls back to updateMany for `eq null` on a unique field", async () => {
-		const update = vi.fn();
-		const updateMany = vi.fn().mockResolvedValue({ count: 1 });
-		const findFirst = vi.fn().mockResolvedValue({ id: "user-id" });
-		const adapter = prismaAdapter(
-			{ $transaction: vi.fn(), user: { findFirst, update, updateMany } },
-			{ provider: "mongodb" },
-		)({
-			user: {
-				additionalFields: {
-					username: { type: "string", unique: true, required: false },
-				},
-			},
-		});
-
-		await adapter.update({
-			model: "user",
-			where: [{ field: "username", operator: "eq", value: null }],
-			update: { name: "Updated" },
-		});
-
-		expect(update).not.toHaveBeenCalled();
-		expect(updateMany).toHaveBeenCalledWith(
-			expect.objectContaining({
-				where: { OR: [{ username: null }, { username: { isSet: false } }] },
-			}),
-		);
-	});
-
 	it("should use update (not updateMany) for insensitive mode on unsupported providers", async () => {
 		const update = vi.fn().mockResolvedValue({
 			id: "user-id",

packages/prisma-adapter/src/prisma-adapter.ts

@@ -219,11 +219,7 @@ export const prismaAdapter = (prisma: PrismaClient, config: PrismaConfig) => {
 						return false;
 					}
 
-					// A unique selector can't be null in Prisma's `WhereUniqueInput`.
-					if (
-						(condition.operator && condition.operator !== "eq") ||
-						condition.value === null
-					) {
+					if (condition.operator && condition.operator !== "eq") {
 						return false;
 					}