My suggestion would be to check the rowcount() first. I generally structure it like this, so you don't assume that all of the nodes have the same tags. You can nest this same structure as dictated by the structure of your XML.
%%[
set @Nodes = BuildRowsetFromXML(@xml,"/collection/items/*",0)
set @rowCount = rowcount(@Nodes)
for @i = 1 to @rowCount do
if rowcount(BuildRowsetFromXML(@xml,concat("/collection/items/item[",@i,"]/var1"),0)) > 0 then
set @var1 = Field(Row(BuildRowsetFromXML(@xml,concat("/collection/items/item[",@i,"]/var1"),0),1),'Value')
endif
]%%
<br>@var1: %%=v(@var1)=%%
%%[ next @i ]%%
