[Swift 6] Fix mutable state error in FirebaseDataEncoder.swift #1579

Job	Run time
spm-package-resolved	1m 11s
pod-lib-lint (ios, macos-14, Xcode_15.2)	6m 39s
pod-lib-lint (ios, macos-15, Xcode_16.2)	1m 46s
pod-lib-lint (tvos, macos-14, Xcode_15.2)	4m 37s
pod-lib-lint (tvos, macos-15, Xcode_16.2)	1m 56s
pod-lib-lint (macos, macos-14, Xcode_15.2)	1m 39s
pod-lib-lint (macos, macos-15, Xcode_16.2)	1m 34s
pod-lib-lint (watchos, macos-14, Xcode_15.2)	1m 9s
pod-lib-lint (watchos, macos-15, Xcode_16.2)	1m 35s
spm (iOS, macos-14, Xcode_15.2)	5m 36s
spm (iOS, macos-15, Xcode_16.2)	2m 10s
spm (tvOS, macos-14, Xcode_15.2)	4m 18s
spm (tvOS, macos-15, Xcode_16.2)	2m 27s
spm (macOS, macos-14, Xcode_15.2)	2m 6s
spm (macOS, macos-15, Xcode_16.2)	1m 44s
spm (catalyst, macos-14, Xcode_15.2)	2m 33s
spm (catalyst, macos-15, Xcode_16.2)	3m 5s
spm (watchOS, macos-14, Xcode_15.2)	2m 49s
spm (watchOS, macos-15, Xcode_16.2)	1m 47s
	50m 41s

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