Ignore transport defaults if SPLIT_KEYBOARD is unset

[zvecr]

Description

No real point in providing a chunk of the split config, on non split keyboards

 qmk info -f json -kb xd84 | grep -A2 split
    "split": {
        "transport": {
            "protocol": "serial"

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).