Fix remaining corner cases after lib_nbgl audit

nbgl_layout_keypad.c: type confusion in nbgl_layoutUpdateHiddenDigits
  When nbActive == nbDigits, children[nbActive] points to the underline
  LINE object, not a digit IMAGE. Reinterpreting and writing its
  foregroundColor field caused silent struct-field corruption. Added a
  type guard that treats a non-IMAGE child as "all digits active" and
  returns 0.

nbgl_layout_keyboard.c: missing NULL checks in nbgl_layoutUpdateKeyboardContent
  mainContainer and its inner text-entry container were dereferenced
  without NULL or type guards. suggestionsContainer (children[1] in the
  KEYBOARD_WITH_SUGGESTIONS path) was also unchecked.

nbgl_use_case.c: missing LOG_WARN before existing NULL return in
  nbgl_useCaseAdvancedChoiceWithDetails
  The bounds check was present but silent; adding the log for consistency
  with all other use-case entry points.

nbgl_use_case_nanos.c: missing LOG_WARN before existing NULL return in
  getContentPage
  Same pattern: p_content == NULL check was present but produced no
  diagnostic.