core: legacyDiffComparisonString don't panic if only deposed objects
core: legacyDiffComparisonString don't panic if only deposed objects

This shim for the benefit of our old tests was not handling the situation
where an InstanceState only contains deposed instances and no current
instance. This is a strange, rare situation but one that does come up in
practice in some odd cases.

luis-silva/terraform,rnaveiras/terraform,gechr/terraform,brandontosch/terraform,ewbankkit/terraform,brandontosch/terraform,heimweh/terraform,VladRassokhin/terraform,mattray/terraform,youhong316/terraform,pdecat/terraform,mattray/terraform,iljaskevic/terraform,rnaveiras/terraform,apparentlymart/terraform,pradeepbhadani/terraform,mattray/terraform,ewbankkit/terraform,apparentlymart/terraform,nlamirault/terraform,tpounds/terraform,luis-silva/terraform,pradeepbhadani/terraform,drebes/terraform,pdecat/terraform,VladRassokhin/terraform,tommynsong/terraform,heimweh/terraform,tpounds/terraform,pryorda/terraform,youhong316/terraform,pdecat/terraform,iljaskevic/terraform,apparentlymart/terraform,gechr/terraform,brandontosch/terraform,hashicorp/terraform,heimweh/terraform,drebes/terraform,VladRassokhin/terraform,tpounds/terraform,nlamirault/terraform,tpounds/terraform,pdecat/terraform,tommynsong/terraform,ewbankkit/terraform,iljaskevic/terraform,ewbankkit/terraform,gechr/terraform,iljaskevic/terraform,heimweh/terraform,rnaveiras/terraform,nlamirault/terraform,apparentlymart/terraform,drebes/terraform,youhong316/terraform,apparentlymart/terraform,pradeepbhadani/terraform,brandontosch/terraform,luis-silva/terraform,youhong316/terraform,drebes/terraform,brandontosch/terraform,mattray/terraform,tommynsong/terraform,hashicorp/terraform,heimweh/terraform,pryorda/terraform,luis-silva/terraform,hashicorp/terraform,VladRassokhin/terraform,pryorda/terraform