Add 'Level-2 provider' label to provider dropdown (#437)

* Add 'Level-2 provider' label to provider dropdown

- Added InputLabel component to the level-2 provider dropdown
- Label appears in both Create Provider and Edit Provider forms
- Improves UI clarity for users selecting level-2 providers

* Fix Level-2 provider label display

- Add id prop to InputLabel for proper Material-UI linking
- Add labelId and label props to Select component
- Ensures label displays correctly with outlined variant styling
- Follows Material-UI pattern used elsewhere in the codebase

* Remove displayEmpty to fix label overlap issue

- Changed value from null to empty string for empty state
- Removed displayEmpty prop which caused label overlap
- Changed MenuItem value from null to empty string
- Label now displays correctly above the select field

Author: vpaturet

src/modals/ModalEditProvider.jsx

@@ -19,7 +19,7 @@ import { Dialog, DialogTitle, DialogContent, DialogActions } from '@mui/material
 import withAuth from 'utils/withAuth';
 import TextField from '@mui/material/TextField';
 import { Checkbox, FormControlLabel } from '@mui/material';
-import { FormControl, Select, MenuItem } from '@mui/material';
+import { FormControl, Select, MenuItem, InputLabel } from '@mui/material';
 import Button from '@mui/material/Button';
 import { connect } from 'react-redux';
 import SuppliersActions from 'actions/SuppliersActions';
@@ -289,13 +289,15 @@ class ModalEditProvider extends Component {
           </div>
           <div style={rowStyle}>
             <FormControl style={{ flex: 1 }}>
+              <InputLabel id="level2-provider-label">Level-2 provider</InputLabel>
               <Select
-                value={this.state.form._migrateDataToProvider}
+                labelId="level2-provider-label"
+                label="Level-2 provider"
+                value={this.state.form._migrateDataToProvider || ''}
                 onChange={e => this.handleChange('_migrateDataToProvider', e.target.value)}
-                displayEmpty
               >
-                <MenuItem value={null} style={{ fontStyle: 'italic' }}>
-                  None
+                <MenuItem value="">
+                  <em>None</em>
                 </MenuItem>
                 {providers
                   .filter(provider => !provider.chouetteInfo.migrateDataToProvider)