buildflags: skip empty cache entries when parsing

Broken in 11c84973ef. The section to skip
an empty input was accidentally removed when some code was refactored to
fix a separate issue.

This skips empty cache entries which allows disabling the `cache-from` and
`cache-to` entries from the command line overrides.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

util/buildflags/cache.go

@@ -175,6 +175,10 @@ func ParseCacheEntry(in []string) (CacheOptions, error) {
 
 	opts := make(CacheOptions, 0, len(in))
 	for _, in := range in {
+		if in == "" {
+			continue
+		}
+
 		if !strings.Contains(in, "=") {
 			// This is ref only format. Each field in the CSV is its own entry.
 			fields, err := csvvalue.Fields(in, nil)