Pasting cell entries into one cell and deleting empty cells

I have the following problem: I have a table with many empty cells. But the first column does not contain empty cells. Like this:

structure(list(variable = c("variable", "Mean (M)", "Standard", 
"deviation", "(SD)", "1. Challenge"), M = c("M", "", "", "", 
"", "3.06"), SD = c("SD", "", "", "", "", "1.08"), X1 = c("1", 
"3.03", "1.09", "", "", ""), X2 = c("2", "2.19", "1.07", "", 
"", "â\210’0.06"), X3 = c("3", "1.93", "1.10", "", "", "0.52***"
), X4 = c("4", "1.86", "1.04", "", "", "â\210’0.14*")), row.names = c(NA, 
6L), class = "data.frame"))

How it looks like now

Now I would like to paste the entries of the two cells under Standard (= "deviation" and "(SD)") into on cell and delete these rows containing only empty cells. So the output should look like this:

structure(list(variable = structure(c(2L, 3L, 1L), .Label = c("1. Challenge", 
"Mean (M)", "Standard deviation SD", "variable"), class = "factor"), 
    M = structure(c(1L, 1L, 2L), .Label = c("", "3.06", "M"), class = "factor"), 
    SD = structure(c(1L, 1L, 2L), .Label = c("", "1.08", "SD"
    ), class = "factor"), `1` = structure(c(4L, 3L, 1L), .Label = c("", 
    "1", "1.09", "3.03"), class = "factor"), `2` = structure(c(3L, 
    1L, 4L), .Label = c("1.07", "2", "2.19", "â\210’0.06"), class = "factor"), 
    `3` = structure(3:1, .Label = c("0.52***", "1.10", "1.93", 
    "3"), class = "factor"), `4` = structure(c(2L, 1L, 4L), .Label = c("1.04", 
    "1.86", "4", "â\210’0.14*"), class = "factor")), row.names = 2:4, class = "data.frame")

How I would like it to look like

Can you help? Thanks!