Minor optimizations on MooseVariableData

[GiudGiud]

locally I observed 8% speedup on simple_diffusion.i with -r 7.
I am hoping someone can reproduce so we can merge this.
I doubt the test suite will show a consistent improvement but let's see with this PR's run

[moosebuild]

Job Documentation, step Docs: sync website on 94117f9 wanted to post the following:

View the site here

This comment will be updated on new commits.

[moosebuild]

Job Coverage, step Generate coverage on 94117f9 wanted to post the following:

Framework coverage

	b45164	#29697 94117f
	Total	Total	+/-	New
Rate	85.25%	85.23%	-0.02%	65.58%
Hits	108039	108022	-17	101
Misses	18699	18726	+27	53

Diff coverage report

Full coverage report

Modules coverage

Coverage did not change

Full coverage reports

Reports

• framework
• chemical_reactions
• combined
• contact
• electromagnetics
• external_petsc_solver
• fluid_properties
• fsi
• functional_expansion_tools
• geochemistry
• heat_transfer
• level_set
• misc
• navier_stokes
• optimization
• peridynamics
• phase_field
• porous_flow
• ray_tracing
• rdg
• reactor
• richards
• scalar_transport
• solid_mechanics
• solid_properties
• stochastic_tools
• thermal_hydraulics
• xfem

Warnings

• framework new line coverage rate 65.58% is less than the suggested 90.0%

This comment will be updated on new commits.