convert(x, to = "stm") needs warning that a zero-count document was dropped

[kbenoit]

The following should report a warning that a zero-count document - here document 3 is dropped. (The output behaviour is correct.) From #696.

mydfm <- as.dfm(matrix(c(1, 0, 2, 0, 
                         0, 0, 1, 2, 
                         0, 0, 0, 0, 
                         1, 2, 3, 4), byrow = TRUE, nrow = 4))
mydfm
# Document-feature matrix of: 4 documents, 4 features (50% sparse).
# 4 x 4 sparse Matrix of class "dfmSparse"
#       features
# docs   feat1 feat2 feat3 feat4
#   doc1     1     0     2     0
#   doc2     0     0     1     2
#   doc3     0     0     0     0
#   doc4     1     2     3     4

convert(mydfm, to = "stm")
# $documents
# $documents$doc1
# [,1] [,2]
# [1,]    1    3
# [2,]    1    2
# 
# $documents$doc2
# [,1] [,2]
# [1,]    3    4
# [2,]    1    2
# 
# $documents$doc4
# [,1] [,2] [,3] [,4]
# [1,]    1    2    3    4
# [2,]    1    2    3    4
# 
# 
# $vocab
# [1] "feat1" "feat2" "feat3" "feat4"
# 
# $meta
# NULL