styled-system 5+ with vue-styled-components - undefined css properties

[carldufourm]

Describe the bug
When using vue-styled-components with styled-system version 5+ and you add styled-system utilities (space for example), you can see that in the CSS of the element you'll have every property of space with a value of undefined. I tested with the latest version 4 (4.2.4) of styled-system and there is no bug.

To Reproduce

• Start a Vue project with packages "vue-styled-components" & "styled-system" (version 5+, this is important)
• Create a styled component with prop types defined
• Put that styled component in a view
• Witness that all of it's prop types are in the CSS with undefined value

With styled-system version 4, no bug
https://codesandbox.io/s/vue-styledsystem-gl9z8

With styled-system version 5, with the bug
https://codesandbox.io/s/vue-styled-system-v3r91

Expected behavior
Since in Vue we need to define all prop types, the css ends up with lots of undefined values. There should be no undefined properties when these properties have no value. When a property has a value, it should end up with the correct value in the style - this part currently works.

Screenshots
Here you can see the undefined properties

[carldufourm]

@liqueflies I opened pull request #102 to fix this bug.

[liqueflies]

Solved merging #111