fix exception for empty content in TikzManager

app/coffee/TikzManager.coffee

@@ -22,8 +22,8 @@ module.exports = TikzManager =
 
 	_includesTikz: (resource) ->
 		# check if we are using tikz externalize
-		content = resource.content.slice(0,65536)
-		if content.indexOf("\\tikzexternalize") >= 0
+		content = resource.content?.slice(0,65536)
+		if content?.indexOf("\\tikzexternalize") >= 0
 			return true
 		else
 			return false

test/unit/coffee/TikzManager.coffee

@@ -30,6 +30,12 @@ describe 'TikzManager', ->
 				{ path: 'output.tex' }
 			]).should.equal false
 
+		it "should return false if the file has no content", ->
+			@TikzManager.needsOutputFile("main.tex", [
+				{ path: 'foo.tex' },
+				{ path: 'main.tex' }
+			]).should.equal false
+
 	describe "injectOutputFile", ->
 		beforeEach ->
 			@rootDir = "/mock"