fix: nmod_mpoly with modulus 1 #2874

Job	Run time
GCC build regression check (no tests)	1m 9s
Alpine Linux, musl, 32-bit (assert, x1.5)	14m 9s
macOS-x86 Clang with BLAS (x2)	14m 15s
macOS-M1, Clang (x3)	8m 52s
MinGW GCC (x0.5)	10m 20s
MSVC (x1)	8m 42s
Nemo.jl (temporary branch)	17m 23s
Ubuntu Clang with examples (x5)	12m 31s
Ubuntu GCC, Code Coverage (x10)	30m 12s
Ubuntu GCC with NTL (assert, x2)	8m 34s
	2h 6m 7s

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