0027561: Since OCCT 7.0.0, exporting a curve to STL creates a file that results in...

0027561: Since OCCT 7.0.0, exporting a curve to STL creates a file that results in an endless loop when read

Add check for empty triangulation when writing STL file to report error instead of creation of empty file.

STL reader has been improved to properly handle case of empty or small files, and Ascii files without EOL at the end.