Fix display of empty files in the Stage view

As JS considers empty strings and "nil"/undefined objects the same, we'd
end up considering empty files as maybe binary, which isn't true.

Author: tiennou

Classes/git/PBGitIndex.m

@@ -559,9 +559,6 @@ - (NSString *)diffForFile:(PBChangedFile *)file staged:(BOOL)staged contextLines
 		NSString *contents = [NSString stringWithContentsOfURL:fileURL
                                                   usedEncoding:&encoding
                                                          error:&error];
-		if (error)
-			return nil;
-
 		return contents;
 	}
 

Resources/html/views/commit/commit.js

@@ -9,9 +9,12 @@ var showNewFile = function(file)
 	diff.innerHTML = "";
 
 	var contents = Index.diffForFile_staged_contextLines_(file, false, contextLines);
-	if (!contents) {
+	if (contents === undefined) {
 		notify("Can not display changes (Binary file?)", -1);
 		return;
+	} else if (contents.length == 0) {
+		notify("Empty file", 1);
+		return;
 	}
 
 	var pre = document.createElement("pre");