PY-16771 Never return null as description in section based docstrings

python/psi-api/src/com/jetbrains/python/psi/StructuredDocString.java

@@ -27,6 +27,7 @@ import java.util.List;
 public interface StructuredDocString {
 
   String getSummary();
+  @NotNull
   String getDescription(); // for formatter
 
   @NotNull

python/src/com/jetbrains/python/documentation/docstrings/EpydocString.java

@@ -50,6 +50,7 @@ public class EpydocString extends TagBasedDocString {
     super(docstringText, "@");
   }
 
+  @NotNull
   @Override
   public String getDescription() {
     final String html = inlineMarkupToHTML(myDescription);

python/src/com/jetbrains/python/documentation/docstrings/PlainDocString.java

@@ -57,6 +57,7 @@ public class PlainDocString extends DocStringLineParser implements StructuredDoc
     return mySummary;
   }
 
+  @NotNull
   @Override
   public String getDescription() {
     return myDescription;

python/src/com/jetbrains/python/documentation/docstrings/SectionBasedDocString.java

@@ -299,9 +299,10 @@ public abstract class SectionBasedDocString extends DocStringLineParser implemen
     return mySummary != null ? mySummary.concatTrimmedLines("\n") : "";
   }
 
+  @NotNull
   @Override
   public String getDescription() {
-    return null;
+    return "";
   }
 
   @NotNull

python/src/com/jetbrains/python/documentation/docstrings/SphinxDocString.java

@@ -123,6 +123,7 @@ public class SphinxDocString extends TagBasedDocString {
     return paramName == null ? getTagValue("type") : getTagValue("type", paramName);
   }
 
+  @NotNull
   @Override
   public String getDescription() {
     return myDescription.replaceAll("\n", "<br/>");

python/src/com/jetbrains/python/documentation/docstrings/TagBasedDocString.java

@@ -74,6 +74,7 @@ public abstract class TagBasedDocString extends DocStringLineParser implements S
 
   public abstract List<String> getAdditionalTags();
 
+  @NotNull
   @Override
   public String getDescription() {
     return myDescription;