[Python] Fix JSON schema for experimental/python (#2761)

## Changes
Make `experimental/python/mutators` and `experimental/python/resources`
fields optional in the JSON schema.

## Why
The JSON schema wasn't correctly reflecting their optionality.

Author: kanterov

NEXT_CHANGELOG.md

@@ -10,5 +10,6 @@
 
 ### Bundles
 * Raise an error when Unity Catalog volumes are used for paths other than artifacts ([#2754](https://github.com/databricks/cli/pull/2754))
+* Made `experimental/python/mutators` and `experimental/python/resources` fields optional in JSON schema ([#2761](https://github.com/databricks/cli/pull/2761))
 
 ### API Changes

acceptance/bundle/python/restricted-execution/output.txt

@@ -4,7 +4,6 @@
 {
   "pydabs": {},
   "python": {
-    "resources": null,
     "mutators": [
       "mutators:read_envs"
     ]

bundle/config/experimental.go

@@ -39,12 +39,12 @@ type Python struct {
 	// defined in Python code.
 	//
 	// Example: ["my_project.resources:load_resources"]
-	Resources []string `json:"resources"`
+	Resources []string `json:"resources,omitempty"`
 
 	// Mutators contains a list of fully qualified function paths to mutator functions.
 	//
 	// Example: ["my_project.mutators:add_default_cluster"]
-	Mutators []string `json:"mutators"`
+	Mutators []string `json:"mutators,omitempty"`
 
 	// VEnvPath is path to the virtual environment.
 	//