fix: allow Joi.any().valid(null) to generate null

Author: nlf

lib/build/types.ts

@@ -79,13 +79,13 @@ export function generateType(schema: Schema, options?: Options): TypeNode {
   }
 
   let valids: TypeNode[] = Array.from((schema as ExtendedSchema)._valids?._values ?? new Set()).map((value) => {
-    // unknown | null is redundant, so just drop the null
-    if (schema.type === "any" && value === null) {
+    // unknown | null is redundant, so just drop the null unless this is the only allowed value
+    if (schema._flags.only !== true && schema.type === "any" && value === null) {
       return;
     }
 
     // string | "" is redundant, so just drop the ""
-    if (schema.type === "string" && value === "") {
+    if (schema._flags.only !== true && schema.type === "string" && value === "") {
       return;
     }