Fix sphinx Makefile on forks where vars.DOCUMENTATION_DOMAIN is not…

… provisioned

sphinx/Makefile

@@ -1,9 +1,14 @@
 # Minimal makefile for Sphinx documentation
 #
 
+# Ensures SPHINX_DOMAIN is not an empty string
+ifeq ($(SPHINX_DOMAIN),)
+	SPHINX_DOMAIN := skore.probabl.ai
+endif
+
 export SPHINX_VERSION ?= dev
 export SPHINX_RELEASE ?= 0.0.0+dev
-export SPHINX_DOMAIN ?= skore.probabl.ai
+export SPHINX_DOMAIN
 
 # You can set these variables from the command line, and also
 # from the environment for the first two.
@@ -21,4 +26,4 @@ help:
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)