Ruby: Include empty StmtSequences in CFG

Empty StmtSequences appear, for example, in the `else` branch of `if`
statements like the following:

    foo

    if cond
      bar
    else
    end

    baz

Before this change, the CFG for this code would look like this:

        foo
         │
         │
         ▼
        cond
         │
    true │
         ▼
        bar
         │
         │
         ▼
         if
         │
         │
         ▼
        baz

i.e. there is linear flow through the condition, the `then` branch, and
out of the if. This doesn't account for the possibility that the
condition is false and `bar` is not executed. After this change, the CFG
looks like this:

          foo
           │
           │
           ▼
          cond
         │    │
    true │    │ false
         ▼    │
        bar   │
         │    │
         │    │
         ▼    ▼
           if
           │
           │
           ▼
          baz

i.e. we correctly account for the `false` condition.

ruby/ql/lib/codeql/ruby/controlflow/internal/ControlFlowGraphImpl.qll

@@ -932,7 +932,13 @@ module Trees {
   class StmtSequenceTree extends PostOrderTree, StmtSequence {
     override predicate propagatesAbnormal(AstNode child) { child = this.getAStmt() }
 
-    override predicate first(AstNode first) { first(this.getStmt(0), first) }
+    override predicate first(AstNode first) {
+      // If this sequence contains any statements, go to the first one.
+      first(this.getStmt(0), first)
+      or
+      // Otherwise, treat this node as a leaf node.
+      not exists(this.getStmt(0)) and first = this
+    }
 
     /** Gets the `i`th child in the body of this body statement. */
     AstNode getBodyChild(int i, boolean rescuable) {

ruby/ql/test/library-tests/controlflow/graph/Cfg.expected

@@ -3675,6 +3675,7 @@ ifs.rb:
 #-----|  -> self
 
 #   47| b
+#-----| false -> else ...
 #-----| true -> self
 
 #   47| then ...
@@ -3689,6 +3690,9 @@ ifs.rb:
 #   48| "true"
 #-----|  -> call to puts
 
+#   49| else ...
+#-----|  -> if ...
+
 #   51| call to puts
 #-----|  -> exit empty_else (normal)
 
@@ -4006,6 +4010,7 @@ loops.rb:
 #-----|  -> exit m4 (normal)
 
 #   31| ... < ...
+#-----| true -> do ...
 #-----| false -> while ...
 
 #   31| x
@@ -4014,6 +4019,9 @@ loops.rb:
 #   31| y
 #-----|  -> ... < ...
 
+#   31| do ...
+#-----|  -> x
+
 raise.rb:
 #    1| enter raise.rb
 #-----|  -> ExceptionA
@@ -5116,9 +5124,20 @@ raise.rb:
 #  147| [ensure: raise] 3
 #-----|  -> [ensure: raise] return
 
+#  150| enter m13
+#-----|  -> ensure ...
+
 #  150| m13
 #-----|  -> m14
 
+#  150| exit m13
+
+#  150| exit m13 (normal)
+#-----|  -> exit m13
+
+#  151| ensure ...
+#-----|  -> exit m13 (normal)
+
 #  154| enter m14
 #-----|  -> element