Adapt for corrected nullability annotation

The return value of `implicit operator string(StringValues values)` has changed to nullable in .NET 8. However the implementation did not change. So adapt our code to use `ToString()` instead, which never returns null.

Author: bkoelman

src/JsonApiDotNetCore/QueryStrings/IncludeQueryStringParameterReader.cs

@@ -51,7 +51,7 @@ public virtual void Read(string parameterName, StringValues parameterValue)
         }
         catch (QueryParseException exception)
         {
-            string specificMessage = exception.GetMessageWithPosition(parameterValue);
+            string specificMessage = exception.GetMessageWithPosition(parameterValue.ToString());
             throw new InvalidQueryStringParameterException(parameterName, "The specified include is invalid.", specificMessage, exception);
         }
     }

src/JsonApiDotNetCore/QueryStrings/PaginationQueryStringParameterReader.cs

@@ -80,7 +80,7 @@ public virtual void Read(string parameterName, StringValues parameterValue)
         }
         catch (QueryParseException exception)
         {
-            string specificMessage = exception.GetMessageWithPosition(isParameterNameValid ? parameterValue : parameterName);
+            string specificMessage = exception.GetMessageWithPosition(isParameterNameValid ? parameterValue.ToString() : parameterName);
             throw new InvalidQueryStringParameterException(parameterName, "The specified pagination is invalid.", specificMessage, exception);
         }
     }

src/JsonApiDotNetCore/QueryStrings/SortQueryStringParameterReader.cs

@@ -65,7 +65,7 @@ public virtual void Read(string parameterName, StringValues parameterValue)
         }
         catch (QueryParseException exception)
         {
-            string specificMessage = exception.GetMessageWithPosition(parameterNameIsValid ? parameterValue : parameterName);
+            string specificMessage = exception.GetMessageWithPosition(parameterNameIsValid ? parameterValue.ToString() : parameterName);
             throw new InvalidQueryStringParameterException(parameterName, "The specified sort is invalid.", specificMessage, exception);
         }
     }

src/JsonApiDotNetCore/QueryStrings/SparseFieldSetQueryStringParameterReader.cs

@@ -68,7 +68,7 @@ public virtual void Read(string parameterName, StringValues parameterValue)
         }
         catch (QueryParseException exception)
         {
-            string specificMessage = exception.GetMessageWithPosition(parameterNameIsValid ? parameterValue : parameterName);
+            string specificMessage = exception.GetMessageWithPosition(parameterNameIsValid ? parameterValue.ToString() : parameterName);
             throw new InvalidQueryStringParameterException(parameterName, "The specified fieldset is invalid.", specificMessage, exception);
         }
     }