Remove redundant empty lines if the parsed file doesn't have system includes

Add unit test for scenario from issue #6.

Author: FastAlien

src/parse/CppFileMerger.ts

@@ -36,7 +36,9 @@ export default class CppFileMerger {
       systemIncludesContent,
       content,
       sourceFilesContent
-    ].join(EOL);
+    ]
+      .filter(content => !!content)
+      .join(EOL);
 
     return removeDoubleEmptyLines(finalContent);
   }

test/data/circularDependencies/expected.cpp

@@ -5,4 +5,3 @@
 // first.hpp
 
 // main.cpp
-

test/data/headerIncludeHeaderInSubdirectory/a.hpp

@@ -0,0 +1,5 @@
+#pragma once
+
+#include "b.hpp"
+
+// a.hpp

test/data/headerIncludeHeaderInSubdirectory/expected.cpp

@@ -0,0 +1,6 @@
+
+// include/b.hpp
+
+// a.hpp
+
+// main.cpp

test/data/headerIncludeHeaderInSubdirectory/include/b.hpp

@@ -0,0 +1,3 @@
+#pragma once
+
+// include/b.hpp

test/data/headerIncludeHeaderInSubdirectory/main.cpp

@@ -0,0 +1,3 @@
+#include "a.hpp"
+
+// main.cpp

test/unit/parse/CppFileMerger.test.ts

@@ -49,3 +49,16 @@ describe("Merging files with circular dependencies", () => {
     expect(content).toEqual(expected);
   });
 });
+
+describe("Merging source with header including another header from include directory", () => {
+  const dataDirectory = "test/data/headerIncludeHeaderInSubdirectory";
+  const merger = new CppFileMerger({
+    includeDirectory: `${dataDirectory}/include`
+  });
+  const content = merger.parse(`${dataDirectory}/main.cpp`);
+
+  test("Generated content equals expected", () => {
+    const expected = fs.readFileSync(`${dataDirectory}/expected.cpp`, "utf-8");
+    expect(content).toEqual(expected);
+  });
+});