server: accept null sampling params (ggml-org#25538)

* server: accept null sampling params

Extend the schema validation to treat a null value as absent, so
clients can send null on nullable params (temperature, top_p, ...)
to request the server default. This matches the OpenAI spec and the
json_value convention used elsewhere.

Add has_field() to skip null in the field eval guards.

* has_field -> has_value​

Author: ServeurpersoCom

tools/server/server-schema.cpp

@@ -568,10 +568,16 @@ static void handle_with_catch(const char * name, std::function<void()> func) {
     }
 }
 
+// treat a null value as absent so clients can send null to request the server default
+static bool has_value(const json & data, const char * n) {
+    auto it = data.find(n);
+    return it != data.end() && !it->is_null();
+}
+
 template <typename T>
 void field_num<T>::eval(field_eval_context & ctx, const json & data) {
     for (const auto & n : name) {
-        if (data.contains(n)) {
+        if (has_value(data, n)) {
             handle_with_catch(n, [&]() {
                 if (custom_handler) {
                 custom_handler(ctx, data);
@@ -593,7 +599,7 @@ void field_num<T>::eval(field_eval_context & ctx, const json & data) {
 void field_str::eval(field_eval_context & ctx, const json & data) {
     GGML_ASSERT(custom_handler);
     for (const auto & n : name) {
-        if (data.contains(n)) {
+        if (has_value(data, n)) {
             handle_with_catch(n, [&]() {
                 custom_handler(ctx, data);
             });
@@ -604,7 +610,7 @@ void field_str::eval(field_eval_context & ctx, const json & data) {
 
 void field_bool::eval(field_eval_context & ctx, const json & data) {
     for (const auto & n : name) {
-        if (data.contains(n)) {
+        if (has_value(data, n)) {
             handle_with_catch(n, [&]() {
                 if (custom_handler) {
                     custom_handler(ctx, data);
@@ -620,7 +626,7 @@ void field_bool::eval(field_eval_context & ctx, const json & data) {
 void field_json::eval(field_eval_context & ctx, const json & data) {
     GGML_ASSERT(custom_handler);
     for (const auto & n : name) {
-        if (data.contains(n)) {
+        if (has_value(data, n)) {
             handle_with_catch(n, [&]() {
                 custom_handler(ctx, data);
             });