fix: do not use the runtime.replace_urls when the runtime is not ready

This happens while generating the Block Structures.

Author: Agrendalath

problem_builder/__init__.py

@@ -1 +1 @@
-__version__ = "5.1.2"
+__version__ = "5.1.3"

problem_builder/mixins.py

@@ -295,8 +295,8 @@ def expand_static_url(self, text):
         if not text:
             return text
 
-        if hasattr(self.runtime, 'replace_urls'):
-            text = self.runtime.replace_urls(text)
+        if replace_urls := getattr(self.runtime, 'replace_urls', None):
+            text = replace_urls(text)
         elif hasattr(self.runtime, 'course_id'):
             # edX Studio uses a different runtime for 'studio_view' than 'student_view',
             # and the 'studio_view' runtime doesn't provide the replace_urls API.