Fixed a bug causing nodes without previews to have zero width

MaterialGraphProject/Assets/UnityShaderEditor/Editor/Drawing/Styles/MaterialGraph.uss

 
 MaterialNodeDrawer #slots {
 	flex-direction: row;
+	width: 200;
 }
 
 MaterialNodeDrawer #slots #input {
 MaterialNodeDrawer #controls {
 	flex-direction: column;
 	padding-bottom: 2;
+	width: 200;
 }
 
 MaterialNodeDrawer #controls #element{