fix(protocol-designer): set default z value for TipPositionModal (#15237

)

Closes RQA-2750

If selecting a custom offset for TipPositionModal on a transfer step,
the Z field should populate with the default offset from bottom rather
than empty.

protocol-designer/src/components/StepEditForm/fields/TipPositionField/TipPositionModal.tsx

@@ -72,7 +72,7 @@ export const TipPositionModal = (
   })
 
   const [zValue, setZValue] = React.useState<string | null>(
-    zSpec?.value == null ? null : String(zSpec?.value)
+    zSpec?.value == null ? String(defaultMmFromBottom) : String(zSpec?.value)
   )
   const [yValue, setYValue] = React.useState<string | null>(
     ySpec?.value == null ? null : String(ySpec?.value)