Remove remaining static references

[schmidt-sebastian]

This changes:

• ResourcePath.EMPTY_PATH to ResourcePath.emptyPath()
• FIeldPath.EMPTYPATH to FIeldPath.emptyPath()
• FIeldPath._DOCUMENT_ID (inlined, only 1 usage)
• DocumentKey.EMPTY ((inlined, only 2 usages)

[google-oss-bot]

Binary Size Report

Affected SDKs

• @firebase/firestore

  Type	Base (acd89ee)	Head (6e15ecb)	Diff
  browser	248 kB	248 kB	+17 B (+0.0%)
  esm2017	194 kB	194 kB	+19 B (+0.0%)
  main	492 kB	492 kB	-25 B (-0.0%)
  module	246 kB	246 kB	+17 B (+0.0%)

• @firebase/firestore/lite

  Type	Base (acd89ee)	Head (6e15ecb)	Diff
  main	140 kB	140 kB	-96 B (-0.1%)

• @firebase/firestore/memory

  Type	Base (acd89ee)	Head (6e15ecb)	Diff
  browser	186 kB	186 kB	+13 B (+0.0%)
  esm2017	145 kB	145 kB	+15 B (+0.0%)
  main	362 kB	362 kB	-27 B (-0.0%)
  module	184 kB	184 kB	+13 B (+0.0%)

• firebase

  Type	Base (acd89ee)	Head (6e15ecb)	Diff
  firebase-firestore.js	287 kB	287 kB	+16 B (+0.0%)
  firebase-firestore.memory.js	226 kB	226 kB	+12 B (+0.0%)
  firebase.js	820 kB	820 kB	+16 B (+0.0%)

Test Logs

• Base (acd89ee4): https://github.com/firebase/firebase-js-sdk/actions/runs/145361187
• Head (6e15ecbd): https://github.com/firebase/firebase-js-sdk/actions/runs/145421774