bar: warn on invalid height type in configuration

[B2krobbery]

Waybar currently accepts invalid values such as "height": null
without emitting any warning, silently falling back to 0.

However, Waybar already emits warnings for other invalid values
(e.g. when height is below the minimum allowed). This creates
inconsistent behavior where some invalid inputs are reported,
while others are silently ignored.

This change ensures that invalid types for height are also
reported with a warning, improving consistency and helping users
identify configuration issues.

Testing

• Set "height": null in config → warning is shown
• Removed height key → no warning
• Set valid value ("height": 22 or higher) → no warning
• Set small value ("height": 20) → existing warning still works