Fix PDF generation failure on blank SVG files.

docs/conf.py

diff --git a/docs/conf.py b/docs/conf.py
index a6dea09..d03c8ae 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -193,8 +193,11 @@
                  r'\DeclareUnicodeCharacter{2220}{\ensuremath{\angle}}' +
                  r'\DeclareUnicodeCharacter{2227}{\ensuremath{\wedge}}' +
                  r'\DeclareUnicodeCharacter{25A1}{\ensuremath{\Box}}' +
-                 r'\DeclareUnicodeCharacter{F06D}{\ensuremath{\mu}}'
+                 r'\DeclareUnicodeCharacter{F06D}{\ensuremath{\Box}}' +
+                 r'\DeclareUnicodeCharacter{F057}{\ensuremath{\otimes}}'
 }
 
+image_converter_args=["-quiet"]
+
 def setup(app):
     app.add_css_file('GF_theme.css')