Do not crash when encountering double semi-colon after package statement

MOE_MIGRATED_REVID=240588095

Author: TimvdLippe

core/src/main/java/com/google/googlejavaformat/java/JavaInputAstVisitor.java

@@ -347,6 +347,7 @@ public Void visitCompilationUnit(CompilationUnitTree node, Void unused) {
       builder.forcedBreak();
       first = false;
     }
+    dropEmptyDeclarations();
     if (!node.getImports().isEmpty()) {
       if (!first) {
         builder.blankLineWanted(BlankLineWanted.YES);

core/src/test/resources/com/google/googlejavaformat/java/testdata/CL240367479.input

@@ -0,0 +1,6 @@
+package foo;;
+
+import com.google.second.Foo;
+import com.google.first.Bar;
+
+public class Blim {}

core/src/test/resources/com/google/googlejavaformat/java/testdata/CL240367479.output

@@ -0,0 +1,6 @@
+package foo;
+;
+import com.google.second.Foo;
+import com.google.first.Bar;
+
+public class Blim {}