fix Material._get_data() for empty groups (regression)
fix Material._get_data() for empty groups (regression)

vlukes/sfepy,lokik/sfepy,lokik/sfepy,BubuLK/sfepy,rc/sfepy,RexFuzzle/sfepy,lokik/sfepy,lokik/sfepy,sfepy/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,RexFuzzle/sfepy,BubuLK/sfepy,sfepy/sfepy,vlukes/sfepy,sfepy/sfepy,rc/sfepy,rc/sfepy,vlukes/sfepy,BubuLK/sfepy