Make the history file optional for the repl

[jneem]

While updating the website, I noticed that the docker instruction docker run --rm -it ghcr.io/tweag/nickel:1.14.0 repl fails because it doesn't have a home directory for the history file. This PR makes the history file optional, so the repl will still run without it.

[github-actions]

Bencher Report

Branch	optional-history
Testbed	ubuntu-latest

Click to view all benchmark results

Benchmark	Latency	microseconds (µs)
diagnostics-benches/inputs/goto-perf.ncl	📈 view plot 🚷 view threshold	8,125.80 µs
diagnostics-benches/inputs/large-record-tree.ncl	📈 view plot 🚷 view threshold	168,110.00 µs
diagnostics-benches/inputs/redis-replication-controller.ncl	📈 view plot 🚷 view threshold	232.71 µs
diagnostics-benches/inputs/small-record-tree.ncl	📈 view plot 🚷 view threshold	376.31 µs
fibonacci 10	📈 view plot 🚷 view threshold	219.00 µs
foldl arrays 50	📈 view plot 🚷 view threshold	419.82 µs
foldl arrays 500	📈 view plot 🚷 view threshold	3,665.20 µs
foldr strings 50	📈 view plot 🚷 view threshold	4,043.90 µs
foldr strings 500	📈 view plot 🚷 view threshold	40,489.00 µs
generate normal 250	📈 view plot 🚷 view threshold	47,562.00 µs
generate normal 50	📈 view plot 🚷 view threshold	1,745.20 µs
generate normal unchecked 1000	📈 view plot 🚷 view threshold	1,893.00 µs
generate normal unchecked 200	📈 view plot 🚷 view threshold	374.39 µs
init-diagnostics-benches/inputs/goto-perf.ncl	📈 view plot 🚷 view threshold	47,460.00 µs
init-diagnostics-benches/inputs/large-record-tree.ncl	📈 view plot 🚷 view threshold	190,530.00 µs
init-diagnostics-benches/inputs/redis-replication-controller.ncl	📈 view plot 🚷 view threshold	42,092.00 µs
init-diagnostics-benches/inputs/small-record-tree.ncl	📈 view plot 🚷 view threshold	42,103.00 µs
pidigits 100	📈 view plot 🚷 view threshold	2,165.00 µs
pipe normal 20	📈 view plot 🚷 view threshold	600.53 µs
pipe normal 200	📈 view plot 🚷 view threshold	5,372.00 µs
product 30	📈 view plot 🚷 view threshold	225.91 µs
requests-benches/inputs/goto-perf.ncl-000	📈 view plot 🚷 view threshold	2,577.80 µs
requests-benches/inputs/large-record-tree.ncl-000	📈 view plot 🚷 view threshold	453,400.00 µs
requests-benches/inputs/large-record-tree.ncl-001	📈 view plot 🚷 view threshold	84.37 µs
scalar 10	📈 view plot 🚷 view threshold	499.80 µs
sum 30	📈 view plot 🚷 view threshold	225.80 µs

🐰 View full continuous benchmarking report in Bencher