Update sibling-index.json

Author: foolip

css/types/sibling-index.json

@@ -14,7 +14,7 @@
             "chrome_android": "mirror",
             "edge": "mirror",
             "firefox": {
-              "version_added": null
+              "version_added": false
             },
             "firefox_android": "mirror",
             "ie": {
@@ -24,7 +24,7 @@
             "opera": "mirror",
             "opera_android": "mirror",
             "safari": {
-              "version_added": null
+              "version_added": false
             },
             "safari_ios": "mirror",
             "samsunginternet_android": "mirror",