Provide error when mixin results in `[object Object]` value

[matthew-dean]

Discussed in #4304

^{Originally posted by uncenter December 31, 2024}
I have a mixin called rgbify, as seen below:

#rgbify(@color) {
  @rgb: red(@color), green(@color), blue(@color);
}

It can be used as color: #rgbify(#fff)[];, and note the [] to capture the return value(s). Without the [] (just #rgbify(#fff)), Less outputs [object Object] as the value for wherever the mixin is used. I most definitely did not mean to have the value be [object Object], and I'm hoping Less could error when it encounters a value like that - otherwise i have no good way of knowing I missed it if I'm not looking carefully/specifically for that mistake.

[matthew-dean]

If I recall, there was supposed to be an evaluation error if evaluating a ruleset as value, so this may be an implementation bug or regression.