fix(Select): display placeholder when value is empty string

The generic Select changes broke placeholder display when form values
are initialized with empty strings. The condition 'value != null'
returns true for empty strings, causing String(value) to display
instead of the placeholder.

Fixed by checking both null/undefined AND empty string conditions.

Fixes failing test in select.stories.tsx that expected placeholder
text to be displayed initially.

Author: codegen-sh[bot]

packages/components/src/ui/select.tsx

@@ -133,7 +133,7 @@ export function Select<T extends React.Key = string>({
           aria-controls={listboxId}
           {...buttonProps}
         >
-          {value != null ? (selectedOption?.label ?? String(value)) : placeholder}
+          {value != null && value !== '' ? (selectedOption?.label ?? String(value)) : placeholder}
           <ChevronIcon className="w-4 h-4 opacity-50" />
         </Trigger>
       </PopoverTrigger>