Don't reference null android.ndkDirectory in build.gradle (#25088)

Summary:
If you (try to) build React Native for Android without having the NDK properly installed and referenced, you get the following error:

>A problem occurred evaluating project ':ReactAndroid'.
\> Cannot get property 'absolutePath' on null object

This is not an overly helpful diagnostic. This PR results in this message instead:

>ndk-build binary cannot be found, check if you've set $ANDROID_NDK environment variable correctly or if ndk.dir is setup in local.properties

Fixes #25087

## Changelog

[Android] [Fixed] - Show proper error message instead of throwing a NullReferenceException if Gradle cannot find the NDK
Pull Request resolved: https://github.com/facebook/react-native/pull/25088

Differential Revision: D15559271

Pulled By: cpojer

fbshipit-source-id: 35c9a9321af4e4a34bf519144ada48884b48352d

ReactAndroid/build.gradle

@@ -193,7 +193,7 @@ def findNdkBuildFullPath() {
     def ndkDir = android.hasProperty("plugin") ? android.plugin.ndkFolder :
             plugins.getPlugin("com.android.library").hasProperty("sdkHandler") ?
                     plugins.getPlugin("com.android.library").sdkHandler.getNdkFolder() :
-                    android.ndkDirectory.absolutePath
+                    android.ndkDirectory ? android.ndkDirectory.absolutePath : null
     if (ndkDir) {
         return new File(ndkDir, getNdkBuildName()).getAbsolutePath()
     }