[TS+Java] Ensure we don't try to add a reference from null.

At least in `Event::PaymentPathFailed` if we try to map an event
with no `retry` we'll hit a `NullPointerException` as we'll try to
add a reference from the `null` retry back to the `Event` itself.

The simple fix is to simply exhaustively check for `null` before
adding references everywhere.

Author: TheBlueMatt

java_strings.py

@@ -704,7 +704,7 @@ def set_null_skip_free(self, var):
         return var + ".ptr" + " = 0;"
 
     def add_ref(self, holder, referent):
-        return holder + ".ptrs_to.add(" + referent + ")"
+        return "if (" + holder + " != null) { " + holder + ".ptrs_to.add(" + referent + "); }"
 
     def fully_qualified_hu_ty_path(self, ty):
         if ty.java_fn_ty_arg.startswith("L") and ty.java_fn_ty_arg.endswith(";"):

typescript_strings.py

@@ -348,7 +348,7 @@ def __init__(self, DEBUG: bool, target: Target, outdir: str, **kwargs):
 	// In TypeScript, protected means "any subclass can access parent fields on instances of itself"
 	// To work around this, we add accessors for other instances' protected fields here.
 	protected static add_ref_from(holder: CommonBase, referent: object) {
-		holder.ptrs_to.push(referent);
+		if (holder !== null) { holder.ptrs_to.push(referent); }
 	}
 	protected static get_ptr_of(o: CommonBase) {
 		return o.ptr;