fix(content-type): prevent ClassCastException on empty binary field in related contentlet (#35188)

[dsilvam]

Summary

• Root cause: When a contentlet with an empty binary field is used as related content at depth≥1, BinaryViewStrategy unconditionally wrote emptyMap() into the contentlet's backing map under the bare field variable key (when AVOID_MAP_SUFFIX_FOR_VIEWS is set). A subsequent DefaultTransformStrategy.addBinaries call then invoked Contentlet.getBinary(), which tried to cast emptyMap() to File, throwing a ClassCastException wrapped as DotDataException: null.
• Fix (root cause): BinaryViewStrategy now skips the map.put when transform(field, contentlet) returns an empty map — nothing is written for fields with no uploaded file.
• Fix (defensive): Contentlet.getBinary() now uses an instanceof File guard before casting, so any future map poisoning cannot cause a ClassCastException.
• Tests: Two new unit tests — BinaryViewStrategyTest (2 cases) and a new case in ContentletTest — covering both fixes. All 5 tests pass.

Test plan

• Existing ContentletTransformerTest (27 tests) — all pass
• New BinaryViewStrategyTest — 2 tests pass
• New ContentletTest#testGetBinary_whenMapContainsNonFileValue_returnsNullWithoutException — passes
• Manual: Page API GET /api/v1/page/json/{page}?depth=1 with a related contentlet that has an empty binary field — no ClassCastException / DotDataException: null in logs

Closes #35188

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