Fix impossible null comparison - identified by linter (#169)

diff --git a/src/main/java/com/google/devtools/build/skydoc/renderer/RendererMain.java b/src/main/java/com/google/devtools/build/skydoc/renderer/RendererMain.java
index 3d7d642..39b11df 100644
--- a/src/main/java/com/google/devtools/build/skydoc/renderer/RendererMain.java
+++ b/src/main/java/com/google/devtools/build/skydoc/renderer/RendererMain.java
@@ -81,7 +81,7 @@
               rendererOptions.aspectTemplateFilePath,
               rendererOptions.repositoryRuleTemplateFilePath,
               rendererOptions.moduleExtensionTemplateFilePath,
-              moduleInfo.getFile() != null ? moduleInfo.getFile() : "...");
+              !moduleInfo.getFile().isEmpty() ? moduleInfo.getFile() : "...");
 
       printWriter.println(renderer.renderMarkdownHeader(moduleInfo));
       printRuleInfos(printWriter, renderer, moduleInfo.getRuleInfoList());