[DataGridPremium] Fix leafField to have correct focus value

[MBilalShafi]

Cherry pick #7950

Before: https://codesandbox.io/s/hasfocus-for-row-grouping-leaves-tuw57o?file=/demo.tsx
After (v5): https://codesandbox.io/s/hasfocus-for-row-grouping-leaves-forked-61g5z6

[mui-bot]

These are the results for the performance tests:

Test case	Unit	Min	Max	Median	Mean	σ
Filter 100k rows	ms	618.5	1,166.2	618.5	873.64	227.24
Sort 100k rows	ms	798.3	1,306.2	1,084.7	1,055.84	163.026
Select 100k rows	ms	273.6	381.4	288.7	304.62	39.186
Deselect 100k rows	ms	173.4	366.2	220	245.48	65.278

Generated by 🚫 dangerJS against 6b2d796