Fix builds without NDK path env set

Summary:
Gradle cannot find the property, because it is defined conditionally depending on env variable.
We want the default for it to be null instead, so the variable exists but doesn't change default NDK location.

Changelog: [Internal]

Reviewed By: motiz88

Differential Revision: D29634984

fbshipit-source-id: 4cf40a1c8d20b2f9008a637456bf9d228d2959d3

build.gradle.kts

@@ -36,7 +36,8 @@ allprojects {
         mavenCentral()
     }
 
-    // used to override ndk path on CI
+    // used to override ndk path/version from env variables on CI
+    ext["ANDROID_NDK_PATH"] = null
     if (System.getenv("LOCAL_ANDROID_NDK_VERSION") != null) {
         setProperty("ANDROID_NDK_VERSION", System.getenv("LOCAL_ANDROID_NDK_VERSION"))
         ext["ANDROID_NDK_PATH"] =  System.getenv("ANDROID_NDK")