analyzer: ensure .dot output is valid for an empty BB

analyzer: ensure .dot output is valid for an empty BB

This patch fixes an issue with the output of -fdump-analyzer-supergraph
on BBs with no statements, where the resulting files were unreadable by
dot e.g.:

Error: syntax error in line 1
... <TABLE BORDER="0"></TABLE> ...
in label of node node_10

gcc/analyzer/ChangeLog:
	* supergraph.cc (supernode::dump_dot): Ensure that the TABLE
	element has at least one TR.

gcc/testsuite/ChangeLog:
	* gcc.dg/analyzer/dot-output.c: Add test coverage for a BB with
	no statements.