TC: Allow setting null JSON schema (#4746)

* TC: Allow setting empty JSON schema

* Fix check for llamacpp schema

* Handle grammar/schema for Aphrodite

* Conditional include for guided_json

* Update default presets with null value

* Conditional include into payload for grammar/schema

default/content/presets/textgen/Default.json

@@ -44,7 +44,7 @@
     "guidance_scale": 1,
     "negative_prompt": "",
     "grammar_string": "",
-    "json_schema": {},
+    "json_schema": null,
     "banned_tokens": "",
     "sampler_priority": [
         "repetition_penalty",

default/content/presets/textgen/Deterministic.json

@@ -44,7 +44,7 @@
     "guidance_scale": 1,
     "negative_prompt": "",
     "grammar_string": "",
-    "json_schema": {},
+    "json_schema": null,
     "banned_tokens": "",
     "sampler_priority": [
         "repetition_penalty",

default/content/presets/textgen/Neutral.json

@@ -44,7 +44,7 @@
     "guidance_scale": 1,
     "negative_prompt": "",
     "grammar_string": "",
-    "json_schema": {},
+    "json_schema": null,
     "banned_tokens": "",
     "sampler_priority": [
         "repetition_penalty",

default/content/presets/textgen/Universal-Creative.json

@@ -44,7 +44,7 @@
     "guidance_scale": 1,
     "negative_prompt": "",
     "grammar_string": "",
-    "json_schema": {},
+    "json_schema": null,
     "banned_tokens": "",
     "sampler_priority": [
         "repetition_penalty",

default/content/presets/textgen/Universal-Light.json

@@ -44,7 +44,7 @@
     "guidance_scale": 1,
     "negative_prompt": "",
     "grammar_string": "",
-    "json_schema": {},
+    "json_schema": null,
     "banned_tokens": "",
     "sampler_priority": [
         "repetition_penalty",

default/content/presets/textgen/Universal-Super-Creative.json

@@ -44,7 +44,7 @@
     "guidance_scale": 1,
     "negative_prompt": "",
     "grammar_string": "",
-    "json_schema": {},
+    "json_schema": null,
     "banned_tokens": "",
     "sampler_priority": [
         "repetition_penalty",