Fix merge.data.frame to keep column names for empty data frames (Bug 18930)

[aakritixyz]

Fixes Bug 18930: merging empty data frames with merge() was losing column names.

This patch preserves all column names when one or both data frames are empty,
without changing behavior for non-empty merges.

Example test cases:

Merge with one empty data frame

df1 <- data.frame(x = numeric())
df2 <- data.frame(y = 1:2)
merge(df1, df2, all.x = TRUE, all.y = TRUE)
#> Column names: "x" "y"
#> 0 rows

Merge with both empty data frames

df3 <- data.frame(a = numeric(), b = numeric())
df4 <- data.frame(c = numeric(), d = numeric())
merge(df3, df4, all.x = TRUE, all.y = TRUE)
#> Column names: "a" "b" "c" "d"
#> 0 rows