fix: null-safety, whitespace trimming, and comma join for variants/branch headers

Addresses Copilot review on PR #295:
- Guard variants(String) and variants(String[]) against null input
- Treat whitespace-only variant/branch as blank via trim().isEmpty()
- Join multi-variant header with "," for consistency with other SDK headers

Blank input intentionally skips (does not remove) the header: headers are a
shared Stack-level map, so remove() would clear the header for sibling
Entry/Query objects and the Stack-level default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: reeshika-h

src/main/java/com/contentstack/sdk/Entry.java

@@ -1158,7 +1158,7 @@ public Entry includeMetadata() {
      *         </pre>
      */
     public Entry variants(@NotNull String variants) {
-        if (!variants.isEmpty()) {
+        if (variants != null && !variants.trim().isEmpty()) {
             this.headers.put("x-cs-variant-uid", variants.trim());
         }
         return this;
@@ -1178,14 +1178,14 @@ public Entry variants(@NotNull String variants) {
      *         </pre>
      */
     public Entry variants(@NotNull String[] variants) {
-        if (variants.length > 0) {
+        if (variants != null && variants.length > 0) {
             List<String> variantList = new ArrayList<>();
             for (String variant : variants) {
                 if (variant != null && !variant.trim().isEmpty())
                     variantList.add(variant.trim());
             }
             if (!variantList.isEmpty()) {
-                this.headers.put("x-cs-variant-uid", String.join(", ", variantList));
+                this.headers.put("x-cs-variant-uid", String.join(",", variantList));
             }
         }
         return this;
@@ -1230,7 +1230,7 @@ public Entry variants(@NotNull String[] variants, @NotNull String branch) {
     }
 
     private Entry applyBranch(String branch) {
-        if (branch != null && !branch.isEmpty()) {
+        if (branch != null && !branch.trim().isEmpty()) {
             this.headers.put("branch", branch.trim());
         }
         return this;

src/main/java/com/contentstack/sdk/Query.java

@@ -1574,7 +1574,7 @@ public Query includeMetadata() {
      *         </pre>
      */
     public Query variants(@NotNull String variants) {
-        if (!variants.isEmpty()) {
+        if (variants != null && !variants.trim().isEmpty()) {
             this.headers.put("x-cs-variant-uid", variants.trim());
         }
         return this;
@@ -1594,14 +1594,14 @@ public Query variants(@NotNull String variants) {
      *         </pre>
      */
     public Query variants(@NotNull String[] variants) {
-        if (variants.length > 0) {
+        if (variants != null && variants.length > 0) {
             List<String> variantList = new ArrayList<>();
             for (String variant : variants) {
                 if (variant != null && !variant.trim().isEmpty())
                     variantList.add(variant.trim());
             }
             if (!variantList.isEmpty()) {
-                this.headers.put("x-cs-variant-uid", String.join(", ", variantList));
+                this.headers.put("x-cs-variant-uid", String.join(",", variantList));
             }
         }
         return this;
@@ -1646,7 +1646,7 @@ public Query variants(@NotNull String[] variants, @NotNull String branch) {
     }
 
     private Query applyBranch(String branch) {
-        if (branch != null && !branch.isEmpty()) {
+        if (branch != null && !branch.trim().isEmpty()) {
             this.headers.put("branch", branch.trim());
         }
         return this;