Fix build arg check

Fix the check on build args to be the length of the map
and not whether the map is nil. The nil check was causing
the cache layer to not be used as it would give a false
result.

Signed-off-by: Urvashi Mohnani <[email protected]>

imagebuildah/stage_executor.go

@@ -838,7 +838,7 @@ func (s *StageExecutor) Execute(ctx context.Context, base string) (imgID string,
 		// we need to call ib.Run() to correctly put the args together before
 		// determining if a cached layer with the same build args already exists
 		// and that is done in the if block below.
-		if checkForLayers && s.builder.Args == nil {
+		if checkForLayers && len(s.builder.Args) == 0 {
 			cacheID, err = s.intermediateImageExists(ctx, node, addedContentSummary, s.stepRequiresLayer(step))
 			if err != nil {
 				return "", nil, errors.Wrap(err, "error checking if cached image exists from a previous build")