Stop exporting empty variables as ENV_NULL

Localize the encoding of empty variables as ENV_NULL into the universal
variables component, and ensure they are not exported as ENV_NULL.

Fixes #4846

src/common.cpp

@@ -1791,6 +1791,33 @@ bool contains(const wcstring_list_t &list, const wcstring &str) {
     return std::find(list.begin(), list.end(), str) != list.end();
 }
 
+wcstring_list_t split_string(const wcstring &val, wchar_t sep) {
+    wcstring_list_t out;
+    size_t pos = 0, end = val.size();
+    while (pos <= end) {
+        size_t next_pos = val.find(sep, pos);
+        if (next_pos == wcstring::npos) {
+            next_pos = end;
+        }
+        out.emplace_back(val, pos, next_pos - pos);
+        pos = next_pos + 1;  // skip the separator, or skip past the end
+    }
+    return out;
+}
+
+wcstring join_strings(const wcstring_list_t &vals, wchar_t sep) {
+    wcstring result;
+    bool first = true;
+    for (const wcstring &s : vals) {
+        if (!first) {
+            result.push_back(sep);
+        }
+        result.append(s);
+        first = false;
+    }
+    return result;
+}
+
 int create_directory(const wcstring &d) {
     bool ok = false;
     struct stat buf;