Avoid IllegalStateException when descriptors are missing.

A missing descriptor directory has caused an IllegalStateException
when we attempted to write our parse history files. Let's be more
careful about this case and skip this history file if there is no such
directory.

Fixes #26711.

CHANGELOG.md

@@ -7,6 +7,8 @@
  * Minor changes
    - Index relays with no known country code or autonomous system
      number using the special values "xz" and "AS0" respectively.
+   - Avoid running into an IllegalStateException when CollecTor is
+     missing a whole descriptor directory.
 
 
 # Changes in version 6.0-1.14.0 - 2018-05-29

src/main/java/org/torproject/onionoo/updater/DescriptorQueue.java

@@ -108,6 +108,11 @@ class DescriptorQueue {
     if (this.historyFile == null) {
       return;
     }
+    if (null == this.descriptors) {
+      log.debug("Not writing history file {}, because we did not read a single "
+          + "descriptor from {}.", this.historyFile, this.directory);
+      return;
+    }
     SortedMap<String, Long> excludedAndParsedFiles = new TreeMap<>();
     excludedAndParsedFiles.putAll(
         this.descriptorReader.getExcludedFiles());