meson: avoid warning about comparison of bool and string

meson.build:2907: WARNING: Trying to compare values of different types (bool, str) using ==.
The result of this is undefined and will become a hard error in a future Meson release.

meson.build

@@ -2903,8 +2903,9 @@ foreach tuple : [
         ['debug mmap cache'],
 ]
 
-        cond = tuple.get(1, '')
-        if cond == ''
+        if tuple.length() >= 2
+                cond = tuple[1]
+        else
                 ident1 = 'HAVE_' + tuple[0].underscorify().to_upper()
                 ident2 = 'ENABLE_' + tuple[0].underscorify().to_upper()
                 cond = conf.get(ident1, 0) == 1 or conf.get(ident2, 0) == 1