MdeModulePkg/DeviceManagerUiLib: Fix the network device MAC display issue

MdeModulePkg/DeviceManagerUiLib: Fix the network device MAC display issue

v3:
* Add NULL string check.

v2:
* Define new STR_FORM_NETWORK_DEVICE_TITLE_HEAD for L" Network Device "
instead of hard code in the code.

Network device tile (STR_FORM_NETWORK_DEVICE_TITLE) is dynamic adjusted
according the different MAC value. So, the string value shouldn't be treated
as a constant string (Network Device). Otherwise, the display will be
incorrect.

Reproduce: Device Manager->Network Device List, select to enter MAC, then to
press ESC back to previous page, then re-enter, found each enter/ESC operation,
the MAC address display +1.

Cc: Eric Dong <eric.dong@intel.com>
Cc: Ye Ting <ting.ye@intel.com>
Cc: Fu Siyuan <siyuan.fu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Wu Jiaxin <jiaxin.wu@intel.com>
Reviewed-by: Ye Ting <ting.ye@intel.com>
Reviewed-by: Fu Siyuan <siyuan.fu@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>