style(mcp): compare nullable strings through one helper

cppcheck 2.20 (the version the lint job pins) reports
knownConditionTrueFalse on two comparators in src/mcp/mcp.c whose
tie-breakers compare one strcmp result after another, each call with
inline "x ? x : \"\"" fallbacks. Neither function differs between these trees; which of
them is flagged depends on the rest of the lint source list. With
scripts/lint.sh --ci and cppcheck 2.20.0 built from source:

- main itself fails on search_result_cmp ("file_order != 0 is always
  false");
- adding one more language grammar and its sources moves the finding
  to project_record_compare ("by_root != 0 is always false");
- carrying each comparator's order in one variable instead still fails,
  now as redundantAssignment on project_record_compare.

Both comparators now call nullable_strcmp(), which orders NULL like ""
exactly as the inline fallbacks did. lint-cppcheck passes again on
main and with the additional sources. Comparison order and results are
unchanged for every input.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Signed-off-by: Sarlette <282189207+SarletteD@users.noreply.github.com>

Author: SarletteD

src/mcp/mcp.c

@@ -2723,18 +2723,23 @@ static void project_record_clear(mcp_project_record_t *record) {
     memset(record, 0, sizeof(*record));
 }
 
+/* strcmp that orders NULL like "". */
+static int nullable_strcmp(const char *a, const char *b) {
+    return strcmp(a ? a : "", b ? b : "");
+}
+
 static int project_record_compare(const void *left, const void *right) {
     const mcp_project_record_t *a = left;
     const mcp_project_record_t *b = right;
-    int by_name = strcmp(a->name ? a->name : "", b->name ? b->name : "");
+    int by_name = nullable_strcmp(a->name, b->name);
     if (by_name != 0) {
         return by_name;
     }
-    int by_root = strcmp(a->root_path ? a->root_path : "", b->root_path ? b->root_path : "");
+    int by_root = nullable_strcmp(a->root_path, b->root_path);
     if (by_root != 0) {
         return by_root;
     }
-    return strcmp(a->db_file ? a->db_file : "", b->db_file ? b->db_file : "");
+    return nullable_strcmp(a->db_file, b->db_file);
 }
 
 typedef enum {
@@ -12605,12 +12610,11 @@ static int search_result_cmp(const void *a, const void *b) {
     if (score_order != 0) {
         return score_order;
     }
-    int qn_order = strcmp(ra->qualified_name ? ra->qualified_name : "",
-                          rb->qualified_name ? rb->qualified_name : "");
+    int qn_order = nullable_strcmp(ra->qualified_name, rb->qualified_name);
     if (qn_order != 0) {
         return qn_order;
     }
-    int file_order = strcmp(ra->file ? ra->file : "", rb->file ? rb->file : "");
+    int file_order = nullable_strcmp(ra->file, rb->file);
     if (file_order != 0) {
         return file_order;
     }