Return output when rsid do not match as NA

[zx8754]

Could we have an output when there is no match, something like:

LDmatrix(snps = c("rs496202", "rs11147477", "rsxyz"),
         pop = "EUR", r2d = "r2",
         token = LDLINK_TOKEN)

#    RS_number rs496202 rs11147477  rsxyz
# 1   rs496202    1.000      0.202     NA
# 2 rs11147477    0.202      1.000     NA
# 3      rsxyz       NA         NA     NA

This could also help with #4 .