Fix empty sections in cv

Whenever an empty section was encountered we returned `None`
which lead to display artifacts as Quarto tried to interpret
what we actually wanted to display.
Instead, we simply return an empty string which is to be
'displayed' so nothing will be shown.

processing/content.py

@@ -3,7 +3,7 @@ from typing import Any
 
 def summary_to_md(data: dict[str, Any], lang: str = "en", headline: str = ""):
     if "summary" not in data:
-        return
+        return ""
     md = f"{headline}\n\n {data['summary'][lang]}\n\n"
 
     return md
@@ -26,7 +26,7 @@ def experience_to_md(
     subdued_publications: bool = True,
 ):
     if "experience" not in data:
-        return
+        return ""
 
     md = f"{headline}\n\n"
     md += "\\definecolor{publication}{rgb}{0.5,0.5,0.5}\n\n"
@@ -56,7 +56,7 @@ def education_to_md(
     data: dict[str, Any], lang: str = "en", headline: str = "", thesis: bool = True
 ):
     if "education" not in data:
-        return
+        return ""
 
     md = f"{headline}\n\n"
 
@@ -73,7 +73,7 @@ def education_to_md(
 
 def qualifications_to_md(data: dict[str, Any], lang: str = "en", headline: str = ""):
     if "skills" not in data:
-        return
+        return ""
 
     md = f"{headline}\n\n"
     for skillset in data["skills"]: