Fix the sqldiff utility program so that it works for schemas that have
tables with zero-length column names.

FossilOrigin-Name: 64263ccb8f9835c615242de6e28db24f953422fa
diff --git a/tool/sqldiff.c b/tool/sqldiff.c
index 0f406d8..56ff53e 100644
--- a/tool/sqldiff.c
+++ b/tool/sqldiff.c
@@ -155,6 +155,7 @@
     "WITH", "WITHOUT",
   };
   int lwr, upr, mid, c, i, x;
+  if( zId[0]==0 ) return sqlite3_mprintf("\"\"");
   for(i=x=0; (c = zId[i])!=0; i++){
     if( !isalpha(c) && c!='_' ){
       if( i>0 && isdigit(c) ){