Align string member empty needle semantics (#1084)

## Motivation

`std.member` accepts either an array or a string as its first argument.
For strings, the official Jsonnet stdlib defines membership through
`std.findSubstr(x, arr)`, and `std.findSubstr("", "abc")` returns an
empty array. That means an empty string should not be considered a
string member.

sjsonnet's native implementation delegated directly to JVM
`String.contains`, which treats the empty string as contained in every
string. This made `std.member("abc", "")` and `std.member("", "")`
return `true`, even though the official stdlib-derived semantics return
`false`.

## Modification

- Require the string needle in `std.member` to be non-empty before
calling `String.contains`.
- Add regression coverage for empty string needles in string membership.
- Add array membership coverage to show that `std.member([""], "")`
remains `true`.

## Result

| Implementation | `std.member("abc", "")` | `std.member("", "")` |
`std.member([""], "")` | `std.findSubstr("", "abc")` |
| --- | --- | --- | --- | --- |
| cpp-jsonnet | `false` | `false` | `true` | `[]` |
| go-jsonnet | `true` | `true` | `true` | `[]` |
| jrsonnet | `false` | `false` | `true` | `[]` |
| sjsonnet before this PR | `true` | `true` | `true` | `[]` |
| sjsonnet after this PR | `false` | `false` | `true` | `[]` |

sjsonnet now matches cpp-jsonnet and jrsonnet for string membership with
an empty needle, while preserving normal substring membership and array
element membership.

## References

- Jsonnet stdlib docs for `std.member`:
https://jsonnet.org/ref/stdlib.html#std-member
- Jsonnet stdlib docs for `std.findSubstr`:
https://jsonnet.org/ref/stdlib.html#std-findSubstr
- Official Jsonnet stdlib `member` definition:
https://github.com/google/jsonnet/blob/master/stdlib/std.jsonnet
- go-jsonnet native `member` implementation:
https://github.com/google/go-jsonnet/blob/master/builtins.go

Author: He-Pin

sjsonnet/src/sjsonnet/stdlib/ArrayModule.scala

@@ -574,7 +574,7 @@ object ArrayModule extends AbstractFunctionModule {
               case n                 =>
                 Error.fail("second argument must be a string, got " + n.prettyName)
             }
-            str.str.contains(secondArg)
+            secondArg.nonEmpty && str.str.contains(secondArg)
           case a: Val.Arr =>
             var i = 0
             var found = false

sjsonnet/test/resources/new_test_suite/member_empty_string.jsonnet

@@ -0,0 +1,6 @@
+std.assertEqual(std.member('abc', ''), false) &&
+std.assertEqual(std.member('', ''), false) &&
+std.assertEqual(std.member('', 'a'), false) &&
+std.assertEqual(std.member('abc', 'bc'), true) &&
+std.assertEqual(std.member([''], ''), true) &&
+std.assertEqual(std.member([''], 'a'), false)

sjsonnet/test/resources/new_test_suite/member_empty_string.jsonnet.golden

@@ -0,0 +1 @@
+true