Disable variable picker when no available variables (#9894)

- Disable variable picker when no available variables
(https://discord.com/channels/1130383047699738754/1331236819227906049)
<img width="496" alt="image"
src="https://github.com/user-attachments/assets/e4cb97c8-648c-4bdf-b230-49687113c9e6"
/>

packages/twenty-front/src/modules/workflow/workflow-variables/components/WorkflowVariablesDropdown.tsx

@@ -47,6 +47,8 @@ export const WorkflowVariablesDropdown = ({
     objectNameSingularToSelect,
   });
 
+  const noAvailableVariables = availableVariablesInWorkflowStep.length === 0;
+
   const initialStep =
     availableVariablesInWorkflowStep.length === 1
       ? availableVariablesInWorkflowStep[0]
@@ -72,11 +74,11 @@ export const WorkflowVariablesDropdown = ({
     setSelectedStep(undefined);
   };
 
-  if (disabled === true) {
+  if (disabled === true || noAvailableVariables) {
     return (
       <StyledDropdownVariableButtonContainer
         isUnfolded={isDropdownOpen}
-        disabled={disabled}
+        disabled={true}
         transparentBackground
       >
         <IconVariablePlus
@@ -97,7 +99,6 @@ export const WorkflowVariablesDropdown = ({
       clickableComponent={
         <StyledDropdownVariableButtonContainer
           isUnfolded={isDropdownOpen}
-          disabled={disabled}
           transparentBackground
         >
           <IconVariablePlus size={theme.icon.size.sm} />