If a named style can't be determined, return null

Previously this would return `style` if it couldn't be resolved and
the `style` attribute on the element is set for some reason. Now it
will return `null`, because it tried to resolve the width but was
not able to. This will allow it to fall back to `element` by default,
or not set the width at all.

This closes select2#2935.

Author: kevin-brown

dist/js/select2.amd.full.js

@@ -4045,6 +4045,8 @@ define('select2/core',[
           return matches[1];
         }
       }
+
+      return null;
     }
 
     return method;

dist/js/select2.amd.js

@@ -4045,6 +4045,8 @@ define('select2/core',[
           return matches[1];
         }
       }
+
+      return null;
     }
 
     return method;

dist/js/select2.full.js

@@ -13580,6 +13580,8 @@ define('select2/core',[
           return matches[1];
         }
       }
+
+      return null;
     }
 
     return method;

dist/js/select2.full.min.js

@@ -4473,6 +4473,8 @@ define('select2/core',[
           return matches[1];
         }
       }
+
+      return null;
     }
 
     return method;

dist/js/select2.js

@@ -151,6 +151,8 @@ define([
           return matches[1];
         }
       }
+
+      return null;
     }
 
     return method;