Remove usage of lib-css mixin and replace false values to null

lib-css mixin in Less version is used to avoid adding properties when value (or one of them if it's a list) is false.

Sass does have a built in support for removing properties if value is null.

Usage of lib-css makes code much more unreadable and harder to debug with source maps.
To solve this problem I did some work to remove usage of this mixin making code much more simpler.

Changes

replaces variable values from false or "" (empty string) to null
updates control structures to work with null values
replaces lib-css calls with just property: value

Difference in compiled css from master

CSS compiled from master branch, and this PR is slightly different. Here is diff result:

diff -c '--color=always' styles-upstream/styles.css styles/styles.css
*** styles-upstream/styles.css  2018-12-20 14:41:34.029159135 +0100
--- styles/styles.css   2018-12-21 17:38:21.683899239 +0100
***************
*** 3990,3996 ****

  .popup-pointer:after {
    left: 1px;
!   top: 0;
    border: solid 7px;
    border-color: transparent transparent #aeaeae transparent;
    z-index: 98; }
--- 3990,3996 ----

  .popup-pointer:after {
    left: 1px;
!   top: 0px;
    border: solid 7px;
    border-color: transparent transparent #aeaeae transparent;
    z-index: 98; }
***************
*** 5177,5184 ****
    width: 16px;
    background-repeat: no-repeat;
    content: '';
!   display: inline-block;
!   margin: ""; }

  .braintree-paypal-account:before {
    left: 17px;
--- 5177,5183 ----
    width: 16px;
    background-repeat: no-repeat;
    content: '';
!   display: inline-block; }

  .braintree-paypal-account:before {
    left: 17px;

As you can see the only difference is that one property does have 0px instead of 0. Which is a result of calculation in styles/vendor/magento-ui/_utilities.scss linee 398.

Other difference is that one property is not included with empty string. (I guess it's a bug on master)

Backward incompatible changes

false values in variables will result in generating incorrect CSS. This PR removes those false values, but any custom themes that override variables may result in incorrect results.
list variables containing false/null values will result in incorrect CSS
0 values with units are not converted to unit-less zeros. For example 0px in Sass will be 0px in CSS, while with using lib-css it was resulting in just 0. I believe this could be avoided by using additional plugin for postcss processor.

SnowdogApps / magento2-theme-blank-sass

Remove usage of lib-css mixin and replace false values to null #205