[SPARK-9225] [MLLIB] LDASuite needs unit tests for empty documents

Add unit tests for running LDA with empty documents.
Both EMLDAOptimizer and OnlineLDAOptimizer are tested.

feynmanliang

Author: Meihua Wu <meihuawu@umich.edu>

Closes #7620 from rotationsymmetry/SPARK-9225 and squashes the following commits:

3ed7c88 [Meihua Wu] Incorporate reviewer's further comments
f9432e8 [Meihua Wu] Incorporate reviewer's comments
8e1b9ec [Meihua Wu] Merge remote-tracking branch 'upstream/master' into SPARK-9225
ad55665 [Meihua Wu] Add unit tests for running LDA with empty documents

mllib/src/test/scala/org/apache/spark/mllib/clustering/LDASuite.scala

@@ -390,6 +390,46 @@ class LDASuite extends SparkFunSuite with MLlibTestSparkContext {
     }
   }
 
+  test("EMLDAOptimizer with empty docs") {
+    val vocabSize = 6
+    val emptyDocsArray = Array.fill(6)(Vectors.sparse(vocabSize, Array.empty, Array.empty))
+    val emptyDocs = emptyDocsArray
+      .zipWithIndex.map { case (wordCounts, docId) =>
+        (docId.toLong, wordCounts)
+    }
+    val distributedEmptyDocs = sc.parallelize(emptyDocs, 2)
+
+    val op = new EMLDAOptimizer()
+    val lda = new LDA()
+      .setK(3)
+      .setMaxIterations(5)
+      .setSeed(12345)
+      .setOptimizer(op)
+
+    val model = lda.run(distributedEmptyDocs)
+    assert(model.vocabSize === vocabSize)
+  }
+
+  test("OnlineLDAOptimizer with empty docs") {
+    val vocabSize = 6
+    val emptyDocsArray = Array.fill(6)(Vectors.sparse(vocabSize, Array.empty, Array.empty))
+    val emptyDocs = emptyDocsArray
+      .zipWithIndex.map { case (wordCounts, docId) =>
+        (docId.toLong, wordCounts)
+    }
+    val distributedEmptyDocs = sc.parallelize(emptyDocs, 2)
+
+    val op = new OnlineLDAOptimizer()
+    val lda = new LDA()
+      .setK(3)
+      .setMaxIterations(5)
+      .setSeed(12345)
+      .setOptimizer(op)
+
+    val model = lda.run(distributedEmptyDocs)
+    assert(model.vocabSize === vocabSize)
+  }
+
 }
 
 private[clustering] object LDASuite {