Use vector-friendly comparison for packages argument.

Otherwise, `sparkR.init()` with multiple `sparkPackages` results in this warning:

```
Warning message:
In if (packages != "") { :
  the condition has length > 1 and only the first element will be used
```

Author: trestletech <[email protected]>

Closes apache#7701 from trestletech/compare-packages and squashes the following commits:

72c8b36 [trestletech] Correct function name.
c52db0e [trestletech] Added test for multiple packages.
3aab1a7 [trestletech] Use vector-friendly comparison for packages argument.

(cherry picked from commit 6143234)
Signed-off-by: Shivaram Venkataraman <[email protected]>

R/pkg/R/client.R

@@ -48,7 +48,7 @@ generateSparkSubmitArgs <- function(args, sparkHome, jars, sparkSubmitOpts, pack
     jars <- paste("--jars", jars)
   }
 
-  if (packages != "") {
+  if (!identical(packages, "")) {
     packages <- paste("--packages", packages)
   }
 

R/pkg/inst/tests/test_client.R

@@ -30,3 +30,7 @@ test_that("no package specified doesn't add packages flag", {
   expect_equal(gsub("[[:space:]]", "", args),
                "")
 })
+
+test_that("multiple packages don't produce a warning", {
+  expect_that(generateSparkSubmitArgs("", "", "", "", c("A", "B")), not(gives_warning()))
+})