Replace warnings with empty files (#272)

Right now lots of our tests assert that there's a warning about a
missing file.

Instead, just provide a valid dummy file.

Author: illicitonion

java/gazelle/testdata/annotations/expectedStderr.txt

@@ -0,0 +1 @@
+{"version": "2"}

java/gazelle/testdata/annotations/maven_install.json

@@ -0,0 +1 @@
+{"version": "2"}

java/gazelle/testdata/bin/expectedStderr.txt

@@ -0,0 +1 @@
+{"version": "2"}

java/gazelle/testdata/bin/maven_install.json

@@ -0,0 +1 @@
+{"version": "2"}

java/gazelle/testdata/bin_in_test_tree/expectedStderr.txt

@@ -0,0 +1 @@
+{"version": "2"}