fix: Allow to set false for actions without mixins

[Embraser01]

📝 Description

This allows to easily have "conditional" actions defined in your service (cf. exemple).
The alternative is to wrap it with an object spread or have a "false" mixin handler.

🎯 Relevant issues

I didn't find any issue about that, but I can create one if you prefer.

💎 Type of change

• Bug fix (non-breaking change which fixes an issue)

📜 Example code

export default {
	name: 'test',
	actions: {
		sum: !env.ENABLE_SUM && {
			handler(ctx) {
				return ctx.params.a + ctx.params.b
			}
		}
	}
}

🚦 How Has This Been Tested?

We got the issue on our side and found the mergeSchemaActions method that handles it. I then found a test case to
add this use case to see if I could repro it.
Once reproduced in unit tests, I fixed it in the code.

🏁 Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have commented my code, particularly in hard-to-understand areas