tests: add empty_environment test, fix whitespace in list_file_types

Verify that ag works with no environment variables set.

tests/empty_environment.t

@@ -0,0 +1,9 @@
+Setup:
+
+  $ . $TESTDIR/setup.sh
+  $ printf "hello world\n" >test.txt
+
+Verify ag runs with an empty environment:
+
+  $ env -i $TESTDIR/../ag --noaffinity --nocolor --workers=1 --parallel hello
+  test.txt:1:hello world

tests/list_file_types.t

@@ -29,7 +29,7 @@ Language types are output:
   
     --batch
         .bat  .cmd
-
+  
     --bazel
         .bazel
   
@@ -419,4 +419,4 @@ Language types are output:
   
     --zephir
         .zep
-  
+