bipartite_projection() does not work if `type` vertex attribute not present

[szhorvat]

Describe the bug
bipartite_projection() does not work if type vertex attribute not present, and remove.type is left at the default value of TRUE.

To reproduce

> g<-make_graph(c(1,2, 1,4, 2,3, 4,3), directed=F)

> bipartite_projection(g, types=c(T,F,T,F))
Error in delete_vertex_attr(res[[1]], "type") : 
  No such vertex attribute: type

> bipartite_projection(g, types=c(T,F,T,F), remove.type=F)
$proj1
IGRAPH f2a8bb7 U-W- 2 1 -- 
+ attr: weight (e/n)
+ edge from f2a8bb7:
[1] 1--2

$proj2
IGRAPH 94ba2d5 U-W- 2 1 -- 
+ attr: weight (e/n)
+ edge from 94ba2d5:
[1] 1--2

Version information

igraph 1.5.1