remove "NA" string case for integer64 (#7664)

Author: MichaelChirico

inst/tests/tests.Rraw

@@ -10525,7 +10525,7 @@ if (test_bit64) {
     "+9223372036854775807",  # integer64 max
     "-9223372036854775806","+9223372036854775806",  # 1 below extreme just to check
     "0","-1","1",
-    "NA",NA,
+    NA,
     "-2147483646", # 1 below extreme to check
     "-2147483647", # smallest integer in R
     "-2147483648", # NA_INTEGER == INT_MIN but valid integer64
@@ -10536,7 +10536,7 @@ if (test_bit64) {
     "+2147483649"
   )))
   ans = c('"V1"',"-9223372036854775807","9223372036854775807","-9223372036854775806","9223372036854775806",
-          "0","-1","1","__NA__","__NA__",
+          "0","-1","1","__NA__",
           "-2147483646","-2147483647","-2147483648","-2147483649",
           "2147483646","2147483647","2147483648","2147483649")
   test(1731.1, class(DT[[1L]]), "integer64")