embed: treat uninitialized FS as empty

As described in the FS documentation.

This prevents http.FS and other clients from panicking when the
go:embed directive is missing.

For #43682
Related #43698

Change-Id: Iecf26d229a099e55d24670c3119cd6c6d17ecc6e
Reviewed-on: https://go-review.googlesource.com/c/go/+/283852
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

Author: Jay Conrod

src/embed/embed.go

@@ -244,6 +244,9 @@ func (f FS) lookup(name string) *file {
 	if name == "." {
 		return dotFile
 	}
+	if f.files == nil {
+		return nil
+	}
 
 	// Binary search to find where name would be in the list,
 	// and then check if name is at that position.
@@ -261,6 +264,9 @@ func (f FS) lookup(name string) *file {
 
 // readDir returns the list of files corresponding to the directory dir.
 func (f FS) readDir(dir string) []file {
+	if f.files == nil {
+		return nil
+	}
 	// Binary search to find where dir starts and ends in the list
 	// and then return that slice of the list.
 	files := *f.files

src/embed/internal/embedtest/embed_test.go

@@ -112,3 +112,20 @@ func TestHidden(t *testing.T) {
 	testDir(t, star, "testdata/.hidden",
 		"fortune.txt", "more/") // but not .more or _more
 }
+
+func TestUninitialized(t *testing.T) {
+	var uninitialized embed.FS
+	testDir(t, uninitialized, ".")
+	f, err := uninitialized.Open(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	fi, err := f.Stat()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !fi.IsDir() {
+		t.Errorf("in uninitialized embed.FS, . is not a directory")
+	}
+}