Inline map missing values are strings instead of null

[thatch]

Describe the bug

My read of the yaml spec is that when only a key is present in a mapping, the value should be interpreted as null. While yq does this for block mappings, it assumes empty-string for inline mappings.

Version of yq: v4.45.1
Operating system: mac
Installed via: homebrew

Input Yaml
Concise yaml document(s) (as simple as possible to show the bug, please keep it to 10 lines or less)
data1.yml:

{x}

Command
The command you ran:

echo '{x}' | yq .

Actual behavior

{x: ''}

Expected behavior

{x: null}

Additional context
The expected behavior is also produced by pyyaml.