Fix getIfPresentObjectNonStringTest

Running the `getIfPresentObjectNonStringTest` with a newer bazel version
(0.22.0) failed due to an error-prone check:

  error: [IncompatibleArgumentType] Argument 'path' should not be
  passed to this method. Its type Path is not compatible with the
  required type: String.

    assertThat(flatFileWebSessionCache.getIfPresent(path)).isNull();
                                                    ^
    (see https://errorprone.info/bugpattern/IncompatibleArgumentType)

The updated code of the websession cache used in the HA-plugin already
used an empty Object() instead of a Path-object. This change adapts this
change to this plugin, which fixes the failing test.

Change-Id: I2f383b13a0cb5c421e4326eaac872daf1b0849ef

src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java

diff --git a/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java b/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java
index 038135a..905d6bf 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/websession/flatfile/FlatFileWebSessionCacheTest.java
@@ -106,8 +106,7 @@
 
   @Test
   public void getIfPresentObjectNonStringTest() throws Exception {
-    Path path = dir.resolve(key);
-    assertThat(flatFileWebSessionCache.getIfPresent(path)).isNull();
+    assertThat(flatFileWebSessionCache.getIfPresent(new Object())).isNull();
   }
 
   @Test