sampling : fix temperature check to allow zero temperature

This commit modifies the temperature sampling check to allow a
temperature value of zero. Previously, the check only allowed
positive temperature values, which excluded the valid case of
zero temperature.

The motivation for this is to enable a zero temperature setting which is
also currently causing the following test to fail:
```console
(venv) $ cd tools/server/tests
(venv) $ ./tests.sh unit/test_basic.py::test_load_split_model
```

common/sampling.cpp

@@ -207,7 +207,7 @@ static bool is_sampler_enabled(enum common_sampler_type type, const struct commo
             }
             break;
         case COMMON_SAMPLER_TYPE_TEMPERATURE:
-            if (params.temp <= 0.0f) {
+            if (params.temp < 0.0f) {
                 return false;
             }
             break;