Fix for CRAN, following change of behaviour of is.atomic(NULL)

[richfitz]

This PR fixes the breaking change coming from R:

r85227 | maechler | 2023-09-28 09:38:16 +0200 (Thu, 28 Sep 2023) | 1 line
is.atomic(NULL) now is FALSE

Please see
https://stat.ethz.ch/pipermail/r-devel/2023-September/082892.html for
more information and guidance.

Please correct before 2023-10-14 to safely retain your package on CRAN.

which is producing errors like

     ══ Failed tests ════════════════════════════════════════════════════════════════
     ── Error ('test-interface.R:19:3'): n_history is configurable ──────────────────
     Error in `ir_serialise_expression(eq$delay$default)`: unhandled expression [odin bug]
     Backtrace:
     ▆
     1. └─odin::odin(...) at test-interface.R:19:2
     2. └─odin::odin_(...)
     3. └─odin::odin_parse_(x, options)
     4. └─odin:::ir_parse(x, options, type)
     5. └─odin:::ir_serialise(ret, options$pretty)
     6. └─odin:::ir_serialise_equations(dat$equations)
     7. └─base::lapply(unname(equations), ir_serialise_equation)
     8. └─odin (local) FUN(X[[i]], ...)
     9. └─odin:::ir_serialise_delay_continuous(eq)
     10. └─odin:::ir_serialise_expression(eq$delay$default)

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
R/ir_serialise.R	99.35% <50.00%> (-0.65%)	⬇️

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