Treat literal "null" string as null when reading marketplace CSV (#6826)

* Treat literal "null" string as null when reading marketplace CSV

RecipeMarketplaceReader already normalizes blank CSV values to null,
but does not handle the literal string "null". This can cause
RecipeBundle.version and requestedVersion to become the string "null"
after a CSV round-trip, leading to display artifacts like
"file:///path/to/file.yml:null" in mod config recipes list.

* Rely on univocity default null handling instead of manual isBlank check

Remove setNullValue("") so univocity returns null for empty fields
natively, eliminating the need for the redundant StringUtils.isBlank
check in the reading loop.

* Restore the blank handling

Author: timtebeek

rewrite-core/src/main/java/org/openrewrite/marketplace/RecipeMarketplaceReader.java

@@ -159,7 +159,7 @@ private void readRecipe(@Nullable String[] row, RecipeMarketplace marketplace, L
             String value = row[i];
             if (value != null) {
                 value = value.trim();
-                if (StringUtils.isBlank(value)) {
+                if (StringUtils.isBlank(value) || "null".equals(value)) {
                     value = null;
                 }
             }

rewrite-core/src/test/java/org/openrewrite/marketplace/RecipeMarketplaceReaderTest.java

@@ -350,6 +350,44 @@ void recipeInMultipleCategoriesHasSeparateBundleInstances() {
         assertThat(rtJsListing.getBundle().getRequestedVersion()).isEqualTo(requestedVersion);
     }
 
+    @Test
+    void nullLiteralVersionIsTreatedAsNull() {
+        // When a CSV contains the literal string "null" for version/requestedVersion,
+        // the reader should treat it as null (not the string "null")
+        RecipeMarketplace marketplace = new RecipeMarketplaceReader().fromCsv("""
+          name,displayName,description,category,ecosystem,packageName,requestedVersion,version
+          org.openrewrite.java.cleanup.UnnecessaryParentheses,Remove Unnecessary Parentheses,Removes unnecessary parentheses,Java Cleanup,maven,org.openrewrite:rewrite-java,null,null
+          """);
+
+        RecipeListing listing = marketplace.getAllRecipes().iterator().next();
+        RecipeBundle bundle = listing.getBundle();
+        assertThat(bundle.getRequestedVersion()).isNull();
+        assertThat(bundle.getVersion()).isNull();
+    }
+
+    @Test
+    void roundTripPreservesNullVersions() {
+        // Create a marketplace with null versions, write to CSV, read back,
+        // and verify versions remain null (not the string "null")
+        RecipeMarketplace marketplace = new RecipeMarketplaceReader().fromCsv("""
+          name,displayName,description,category,ecosystem,packageName
+          org.openrewrite.java.cleanup.UnnecessaryParentheses,Remove Unnecessary Parentheses,Removes unnecessary parentheses,Java Cleanup,maven,org.openrewrite:rewrite-java
+          """);
+
+        // Verify initial state has null versions
+        RecipeListing listing = marketplace.getAllRecipes().iterator().next();
+        assertThat(listing.getBundle().getVersion()).isNull();
+        assertThat(listing.getBundle().getRequestedVersion()).isNull();
+
+        // Round-trip through CSV
+        @Language("csv") String csv = new RecipeMarketplaceWriter().toCsv(marketplace);
+        RecipeMarketplace roundTripped = new RecipeMarketplaceReader().fromCsv(csv);
+
+        RecipeListing rtListing = roundTripped.getAllRecipes().iterator().next();
+        assertThat(rtListing.getBundle().getVersion()).isNull();
+        assertThat(rtListing.getBundle().getRequestedVersion()).isNull();
+    }
+
     private void setVersionRecursive(RecipeMarketplace.Category category, String packageName,
                                      String requestedVersion, String version) {
         for (RecipeListing recipe : category.getRecipes()) {