make tree building null safe when an input does not always produce source positions.  When parsing from an event stream serialized from an AST we need to take into account that synthetic nodes do not always have source positions
make tree building null safe when an input does not always produce source positions.  When parsing from an event stream serialized from an AST we need to take into account that synthetic nodes do not always have source positions

mikesamuel/code-interlingua,mikesamuel/code-interlingua