Correct processing when build-args is empty (none). (#356)

In certain scenarios when singularity runner is used, setting `build_args` to empty string (-Psingularity.build_args="") results in error (accessign variable that's None). This commit fixes it.

runners/mlcube_singularity/mlcube_singularity/singularity_run.py

@@ -223,7 +223,7 @@ def configure(self) -> None:
             recipe=s_cfg.build_file,
             image_dir=s_cfg.image_dir,
             image_name=s_cfg.image,
-            build_args=s_cfg.build_args,
+            build_args=s_cfg.build_args or "",
         )
 
     def run(self) -> None: