fixes #95 FilepathGlob("") should return nil

doublestar_test.go

diff --git a/doublestar_test.go b/doublestar_test.go
index 26ff894..bf6414e 100644
--- a/doublestar_test.go
+++ b/doublestar_test.go
@@ -28,6 +28,7 @@
 var onWindows = runtime.GOOS == "windows"
 
 var matchTests = []MatchTest{
+	{"", "", true, false, nil, true, false, true, true, 0, 0},
 	{"*", "", true, true, nil, false, false, true, false, 0, 0},
 	{"*", "/", false, false, nil, false, false, true, false, 0, 0},
 	{"/*", "/", true, true, nil, false, false, true, false, 0, 0},

utils.go

diff --git a/utils.go b/utils.go
index 0ab1dc9..6b8df9a 100644
--- a/utils.go
+++ b/utils.go
@@ -84,6 +84,16 @@
 // filepath.ErrBadPattern.
 //
 func FilepathGlob(pattern string, opts ...GlobOption) (matches []string, err error) {
+	if pattern == "" {
+		// special case to match filepath.Glob behavior
+		g := newGlob(opts...)
+		if g.failOnIOErrors {
+			// match doublestar.Glob behavior here
+			return nil, os.ErrInvalid
+		}
+		return nil, nil
+	}
+
 	pattern = filepath.Clean(pattern)
 	pattern = filepath.ToSlash(pattern)
 	base, f := SplitPattern(pattern)