Don't add final empty line if file has no contents

Fix prettier#60. Use same logic as prettier's printer-estree.js "Program"
case.

Author: ryanashcraft

src/printer/index.js

@@ -472,7 +472,14 @@ function genericPrint(path, options, print) {
 
   switch (n.ast_type) {
     case "Module": {
-      return concat([printBody(path, print, true /*isTopLevel*/), hardline]);
+      const parts = [printBody(path, print, true /*isTopLevel*/)];
+
+      // Only force a trailing newline if there were any contents.
+      if (n.body.length || n.comments) {
+        parts.push(hardline);
+      }
+
+      return concat(parts);
     }
 
     case "AsyncFunctionDef":

tests/python_empty/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,21 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`empty_file.py 1`] = `
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+`;
+
+exports[`empty_file.py 2`] = `
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+`;
+
+exports[`single_space.py 1`] = `
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+`;
+
+exports[`single_space.py 2`] = `
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+`;

tests/python_empty/empty_file.py

@@ -0,0 +1,2 @@
+run_spec(__dirname, ["python"], { pythonVersion: "2" });
+run_spec(__dirname, ["python"], { pythonVersion: "3" });

tests/python_empty/jsfmt.spec.js

@@ -0,0 +1 @@
+