lib/ignore: Don't crash on empty patterns (fixes #6300) (#6301)

Instead of panicking when we try to look closer at the empty pattern,
return something like `invalid pattern "(?d)" in ignore file: missing
pattern`.

lib/ignore/ignore.go

@@ -422,6 +422,10 @@ func parseLine(line string) ([]Pattern, error) {
 		}
 	}
 
+	if line == "" {
+		return nil, errors.New("missing pattern")
+	}
+
 	if pattern.result.IsCaseFolded() {
 		line = strings.ToLower(line)
 	}