Backend less-parser fails on files containing only mixins

[homecoded]

In lib/internal/Magento/Framework/Css/PreProcessor/Adapter/Less/Processor.php:88

if (trim($content) === '') {
    $errorMessage = PHP_EOL . self::ERROR_MESSAGE_PREFIX . PHP_EOL . $path;
    $this->logger->critical($errorMessage);

    throw new ContentProcessorException(new Phrase($errorMessage));
}

... the less processor checks if a file was compiled successfully by checking if the content is not an empty string.
If you include the less version of Twitter-Bootstrap into a theme or extenion the build process fails because of that.

explanation:

Magento tries to build all of the file separately. When it comes across a mixin file the result will be empty because all mixins are resolved to an empty string by the less parser. See vendor/oyejorge/less.php/lib/Less/Visitor/toCSS.php:31

public function visitMixinDefinition($mixinNode){
    // mixin definitions do not get eval'd - this means they keep state
    // so we have to clear that state here so it isn't used if toCSS is called twice
    $mixinNode->frames = array();
    return array();
}

workaround:

A temporary workaround is to add a bogus CSS definition into mixin files like:

#bogus { display: none }    

possible solution:

Empty results of less-files should not be treated as an error. A warning would surely suffice. I can propose a PR if this is a valid solution.

[MomotenkoNatalia]

@homecoded Thanks for reporting this issue. We will do fixes under MAGETWO-55546

[homecoded]

@MomotenkoNatalia Awesome. Thanks.