title: Firebase realtime database always return null on reads
body: <p>I'm trying to write test android-app using firebase backend. 
  I made all settings according to tutorials (using Google sign-in for auth and auth-checking rules for root object in realtime database) but still can not make firebase return something but nulls in snapshot's value (when onDataChange() is called). 
  I'm sure that path and other settings are valid - otherwise I get "Permission Denied" in onCancelled() callback.
  What can I do wrong?</p>
  
  <p>Code:</p>
  
  <pre><code>    mDatabase = FirebaseDatabase.getInstance().getReference("api");
      mDatabase.child("test")
              .addListenerForSingleValueEvent(new ValueEventListener() {
                  @Override
                  public void onDataChange(@NonNull DataSnapshot
                                                   dataSnapshot) {
                      if (dataSnapshot.exists()) {
                          //code that's never called
                      }
                  }
  
                  @Override
                  public void onCancelled(@NonNull DatabaseError
                                                  databaseError) {
                      //log error
                  }
              });
  </code></pre>
  
  <p>Dependecies:</p>
  
  <pre><code>implementation 'com.google.firebase:firebase-core:16.0.4'
  implementation 'com.google.firebase:firebase-database:16.0.3'
  implementation 'com.google.firebase:firebase-auth:16.0.4'
  </code></pre>
  
  <p>Data structure:</p>
  
  <pre><code>{"test" : true}
  </code></pre>
  
  <p>Snapshot's toString() result:</p>
  
  <pre><code>DataSnapshot { key = test, value = null }
  </code></pre>
  
  <p>The only one thing I could retrieve successfully from Firebase (if it does matter) is: </p>
  
  <pre><code>FirebaseDatabase.getInstance().getReference(".info/serverTimeOffset"); 
  </code></pre>
  
  <p>Database rules:</p>
  
  <pre><code>    {"rules": {
  "api":{
    ".read": "auth != null",
    ".write": "auth != null",
  }}}
  </code></pre>
  
  <p>Thanks for any help.</p>