Guard against the assumption children gives nodes

I've recently become aware that some dodgy syntax can result in
numchildren(...) > 0 but no children(...), represented as nothing.

We might as well systematically remove the chance of running into this.

If this isn't the end of the syntax surprises, I'll look at what's
needed to make JET confident there aren't any more lurking around the
corner (this is probably a good idea anyway, but spinning plates and all
that).

Author: tecosaur

src/JuliaSyntaxHighlighting.jl

@@ -215,13 +215,13 @@ function _hl_annotations!(highlights::Vector{@NamedTuple{region::UnitRange{Int},
         :julia_macro
     elseif nkind == K"StringMacroName"; :julia_macro
     elseif nkind == K"CmdMacroName"; :julia_macro
-    elseif nkind == K"::";
+    elseif nkind == K"::"
         if JuliaSyntax.is_trivia(node) || numchildren(node) == 0
             :julia_typedec
         else
             literal_typedecl = findfirst(
-                c ->kind(c) == K"::" && JuliaSyntax.is_trivia(c),
-                children(node))
+                c -> kind(c) == K"::" && JuliaSyntax.is_trivia(c),
+                something(children(node), GreenNode[]))
             if !isnothing(literal_typedecl)
                 shift = sum(c ->Int(span(c)), node[1:literal_typedecl])
                 region = first(region)+shift:last(region)
@@ -260,8 +260,8 @@ function _hl_annotations!(highlights::Vector{@NamedTuple{region::UnitRange{Int},
             :julia_keyword
         else
             literal_where = findfirst(
-                c ->kind(c) == K"where" && JuliaSyntax.is_trivia(c),
-                children(node))
+                c -> kind(c) == K"where" && JuliaSyntax.is_trivia(c),
+                something(children(node), GreenNode[]))
             if !isnothing(literal_where)
                 shift = sum(c ->Int(span(c)), node[1:literal_where])
                 region = first(region)+shift:last(region)
@@ -281,7 +281,7 @@ function _hl_annotations!(highlights::Vector{@NamedTuple{region::UnitRange{Int},
         :julia_broadcast
     elseif nkind in (K"call", K"dotcall") && JuliaSyntax.is_prefix_call(node)
         argoffset, arg1 = 0, nothing
-        for arg in children(node)
+        for arg in something(children(node), GreenNode[])
             argoffset += span(arg)
             if !JuliaSyntax.is_trivia(arg)
                 arg1 = arg
@@ -338,7 +338,7 @@ function _hl_annotations!(highlights::Vector{@NamedTuple{region::UnitRange{Int},
     end
     numchildren(node) == 0 && return
     lnode = node
-    for child in children(node)
+    for child in something(children(node), GreenNode[])
         cctx = HighlightContext(content, offset, lnode, pdepths)
         _hl_annotations!(highlights, GreenLineage(child, lineage), cctx)
         lnode = child