perf: add Doc.Null check to Argument avoids Concat creation

[TimothyMakkison]

Add check for if modifiers are Doc.Null to avoid allocating a Concat and Doc[]

Benchmarks (timing is inaccurate)

Before

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Allocated
Default_CodeFormatter_Tests	131.9 ms	3.30 ms	8.86 ms	128.0 ms	3000.0000	1000.0000	34.57 MB
Default_CodeFormatter_Complex	261.3 ms	5.16 ms	4.03 ms	261.3 ms	5000.0000	2000.0000	53.03 MB

After

Method	Mean	Error	StdDev	Median	Gen0	Gen1	Allocated
Default_CodeFormatter_Tests	189.5 ms	10.25 ms	28.05 ms	195.6 ms	3000.0000	1000.0000	34.31 MB
Default_CodeFormatter_Complex	264.2 ms	5.27 ms	13.97 ms	260.0 ms	5000.0000	2000.0000	52.31 MB

[belav]

LGTM!