tpl: Do not return errors in substr for out-of-bounds cases

Most other substr implementations don't error out in out-of-bounds cases
but simply return an empty string (or a value that's printed as an empty
string). We'll follow their lead and not exit template execution.  Allow
the user decide what to do with the empty result.

Fixes gohugoio#8113

tpl/strings/strings_test.go

@@ -451,9 +451,9 @@ func TestSubstr(t *testing.T) {
 		{"abcdef", -3, 1, "d"},
 		{"abcdef", 0, -1, "abcde"},
 		{"abcdef", 2, -1, "cde"},
-		{"abcdef", 4, -4, false},
-		{"abcdef", 7, 1, false},
-		{"abcdef", 6, nil, false},
+		{"abcdef", 4, -4, ""},
+		{"abcdef", 7, 1, ""},
+		{"abcdef", 6, nil, ""},
 		{"abcdef", 1, 100, "bcdef"},
 		{"abcdef", -100, 3, "abc"},
 		{"abcdef", -3, -1, "de"},