fix(bec_widgets): require sample owner in sample-storage widget

The owner prompt was labeled and treated as optional, but an occupied
slot must always have an owner recorded. Both the rename and new-sample
dialogs now validate the owner the same way the name field already is:
cancelling or leaving it empty aborts the write instead of silently
falling back to a blank/previous owner.

csaxs_bec/bec_widgets/widgets/sample_storage/sample_storage.py

@@ -428,10 +428,17 @@ class SampleStorageWidget(BECWidget, QWidget):
         owner, ok = QInputDialog.getText(
             self,
             "Sample owner",
-            f"Owner for slot {self._slot_title(slot)} (optional):",
+            f"Owner for slot {self._slot_title(slot)}:",
             text=current_owner,
         )
-        owner = owner.strip() if ok else current_owner
+        if not ok:
+            return
+        owner = owner.strip()
+        if not owner:
+            QMessageBox.warning(
+                self, "Invalid owner", "An occupied slot needs a non-empty owner."
+            )
+            return
         if self._write_slot(slot, 1, name, owner):
             self.refresh()
 
@@ -447,9 +454,16 @@ class SampleStorageWidget(BECWidget, QWidget):
             QMessageBox.warning(self, "Invalid name", "Please enter a non-empty sample name.")
             return
         owner, ok = QInputDialog.getText(
-            self, "Sample owner", f"Owner for slot {self._slot_title(slot)} (optional):"
+            self, "Sample owner", f"Owner for slot {self._slot_title(slot)}:"
         )
-        owner = owner.strip() if ok else ""
+        if not ok:
+            return
+        owner = owner.strip()
+        if not owner:
+            QMessageBox.warning(
+                self, "Invalid owner", "An occupied slot needs a non-empty owner."
+            )
+            return
         if self._write_slot(slot, 1, name, owner):
             self.refresh()