build: fix icu-i18n pkg-config version check

The pkg_config() helper can either return a tuple of None values
(no pkg-config installed) and that was what the check was testing
for, but it can also return a tuple of empty strings when the
package isn't installed.

configure.py

@@ -1348,7 +1348,7 @@ def write_config(data, name):
     # ICU from pkg-config.
     o['variables']['v8_enable_i18n_support'] = 1
     pkgicu = pkg_config('icu-i18n')
-    if pkgicu[0] is None:
+    if not pkgicu[0]:
       error('''Could not load pkg-config data for "icu-i18n".
        See above errors or the README.md.''')
     (libs, cflags, libpath, icuversion) = pkgicu