commit: git_commit_create_with_signature should support null signature

If provided with a null signature, skip adding the signature header and create the commit anyway.

include/git2/commit.h

@@ -480,7 +480,8 @@ GIT_EXTERN(int) git_commit_create_buffer(
  *
  * @param out the resulting commit id
  * @param commit_content the content of the unsigned commit object
- * @param signature the signature to add to the commit
+ * @param signature the signature to add to the commit. Leave `NULL`
+ * to create a commit without adding a signature field.
  * @param signature_field which header field should contain this
  * signature. Leave `NULL` for the default of "gpgsig"
  * @return 0 or an error code

src/commit.c

@@ -876,12 +876,15 @@ int git_commit_create_with_signature(
 		return -1;
 	}
 
-	field = signature_field ? signature_field : "gpgsig";
-
 	/* The header ends after the first LF */
 	header_end++;
 	git_buf_put(&commit, commit_content, header_end - commit_content);
+
+	if (signature != NULL) {
+		field = signature_field ? signature_field : "gpgsig";
 		format_header_field(&commit, field, signature);
+	}
+
 	git_buf_puts(&commit, header_end);
 
 	if (git_buf_oom(&commit))