#39 Fix throwing error for attribute selectors without a value

Author: Semigradsky

src/__tests__/index.js

@@ -206,3 +206,10 @@ testThrows(
   'foo{bar:calc(500px/2px)}',
   /Cannot divide by "px", number expected/
 );
+
+test(
+  'should return the same and not thrown an exception for attribute selectors without a value',
+  'button[disabled]{}',
+  'button[disabled]{}',
+  { selectors: true }
+);

src/lib/transform.js

@@ -4,6 +4,10 @@ import reduceCssCalc from 'reduce-css-calc';
 const MATCH_CALC = /((?:\-[a-z]+\-)?calc)/;
 
 function transformValue(value, options, result, item) {
+  if (!value) {
+    return value;
+  }
+
   const reduced = reduceCssCalc(value, options.precision)
   // if the warnWhenCannotResolve option is on, inform the user that the calc
   // expression could not be resolved to a single value