Only use enabled exposure for proxy

The exposure proxy found is only relevant if it's enabled.

Relevant for:
 - histogram UI visualizing
 - currently colorbalancergb makes use of blackpoint and exposure setting in the proxy module.
   This should not happen if module is not enabled.

Author: jenshannoschwalm

src/develop/develop.c

@@ -2873,12 +2873,8 @@ dt_dev_proxy_exposure_t *dt_dev_exposure_proxy_available(dt_develop_t *dev)
   dt_dev_proxy_exposure_t *instance = &dev->proxy.exposure;
   const dt_iop_module_t *exposure = instance && instance->module ? instance->module : NULL;
 
-  // note: can we do better here by testing the proxy module being enabled?
-  // this is relevant for
-  //   - user feedback in histogram visualizing and
-  //   - provided black and exposure data while extracting patches in colorbalancergb
-  // return exposure->enabled ? instance : NULL;
-  return exposure ? instance : NULL;
+  // only report back if proxy exposure module is anabled.
+  return exposure->enabled ? instance : NULL;
 }
 
 float dt_dev_exposure_get_exposure(dt_develop_t *dev)