SecurityPkg/SecureBootConfigDxe: Handle lack of STR_SIGNATURE_* tokens

SecurityPkg/SecureBootConfigDxe: Handle lack of STR_SIGNATURE_* tokens

Add check to avoid NULL ptr dereference. The function HiiGetString
will return NULL pointer when the platform does not install the
appropriate string or call HiiGetString fail.(For example, HII not
support specified language.)

Cc: Zhang Chao <chao.b.zhang@intel.com>
Cc: Wu Hao <hao.a.wu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: chenc2 <chen.a.chen@intel.com>
Reviewed-by: Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by: Wu Hao <hao.a.wu@intel.com>