[ADAM-1186] Mask null from fs.globStatus.

Resolves #1186. Check for a null pointer in ADAMContext.getFiles. Make
ADAMContext.getFsAndFiles package private so that we can unit test it.
Added unit tests to check for proper exception with empty directories and
bad globs.

adam-core/src/main/scala/org/bdgenomics/adam/rdd/ADAMContext.scala

@@ -304,7 +304,7 @@ class ADAMContext private (@transient val sc: SparkContext) extends Serializable
     val paths = if (fs.isDirectory(path)) fs.listStatus(path) else fs.globStatus(path)
 
     // the path must match at least one file
-    if (paths.isEmpty) {
+    if (paths == null || paths.isEmpty) {
       throw new FileNotFoundException(
         s"Couldn't find any files matching ${path.toUri}. If you are trying to" +
           " glob a directory of Parquet files, you need to glob inside the" +
@@ -327,7 +327,7 @@ class ADAMContext private (@transient val sc: SparkContext) extends Serializable
    *
    * @throws FileNotFoundException if the path does not match any files.
    */
-  private def getFsAndFiles(path: Path): Array[Path] = {
+  private[rdd] def getFsAndFiles(path: Path): Array[Path] = {
 
     // get the underlying fs for the file
     val fs = Option(path.getFileSystem(sc.hadoopConfiguration)).getOrElse(

adam-core/src/test/scala/org/bdgenomics/adam/rdd/ADAMContextSuite.scala

@@ -17,7 +17,7 @@
  */
 package org.bdgenomics.adam.rdd
 
-import java.io.File
+import java.io.{ File, FileNotFoundException }
 import java.util.UUID
 import htsjdk.samtools.DiskBasedBAMFileIndex
 import com.google.common.io.Files
@@ -378,8 +378,26 @@ class ADAMContextSuite extends ADAMFunSuite {
     val outputPath = tmpLocation()
     reads.saveAsParquet(outputPath)
     reads.saveAsParquet(outputPath.replace(".adam", ".2.adam"))
+
+    val paths = new Path(outputPath.replace(".adam", "*.adam") + "/*")
+    assert(sc.getFsAndFiles(paths).size > 2)
+
     val reloadedReads = sc.loadParquetAlignments(outputPath.replace(".adam", "*.adam") + "/*")
     assert((2 * reads.rdd.count) === reloadedReads.rdd.count)
   }
+
+  sparkTest("bad glob should fail") {
+    val inputPath = resourcePath("small.sam")
+    intercept[FileNotFoundException] {
+      sc.getFsAndFiles(new Path(inputPath.replace(".sam", "*.sad")))
+    }
+  }
+
+  sparkTest("empty directory should fail") {
+    val outputPath = tmpLocation()
+    intercept[FileNotFoundException] {
+      sc.getFsAndFiles(new Path(outputPath))
+    }
+  }
 }