a11y: Check if path is NULL when removing from cache

a11y: Check if path is NULL when removing from cache

`gtk_at_spi_cache_add_context()` checks if the GtkAtSpiContext's path
is NULL before inserting the context object into the hash table.
Do the same in `gtk_at_spi_cache_remove_context()` to avoid a NULL
pointer dereference in `g_str_hash()` during the hash table lookup
if a context with NULL path is removed. That can happen when the
GtkAtSpiRoot::base_path is NULL, which, in turn, can happen if
`g_application_get_dbus_object_path()` returns NULL.

  ==394047==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7fd1966f8b84 bp 0x7fff11e3ded0 sp 0x7fff11e3de58 T0)
  ==394047==The signal is caused by a READ memory access.
  ==394047==Hint: address points to the zero page.
      #0 0x7fd1966f8b84 in g_str_hash (/usr/lib/libglib-2.0.so.0+0x37b84)
      #1 0x7fd1966f9c09 in g_hash_table_contains (/usr/lib/libglib-2.0.so.0+0x38c09)
      #2 0x7fd196062c10 in gtk_at_spi_cache_remove_context ../gtk/a11y/gtkatspicache.c:447
      #3 0x7fd19606e0a9 in gtk_at_spi_root_unregister ../gtk/a11y/gtkatspiroot.c:653
      #4 0x7fd196067f58 in gtk_at_spi_context_unrealize ../gtk/a11y/gtkatspicontext.c:1559
      #5 0x7fd195ced97f in gtk_at_context_unrealize ../gtk/gtkatcontext.c:668
      #6 0x7fd195f5576e in gtk_widget_unroot_at_context ../gtk/gtkwidget.c:2399
      #7 0x7fd195f55bd2 in gtk_widget_unroot ../gtk/gtkwidget.c:2499
      ...