fix(core): guard null input in ToolValidator.validateInput to prevent NPE

[1143506050]

When a ToolUseBlock has parsed arguments in its input map but no raw
content string, ToolExecutor.executeCore passes null to
ToolValidator.validateInput(), which forwards it to networknt-schema's
validate(null, …). networknt-schema then throws:

  NullPointerException: argument "content" is null

This causes tool calls to fail with a misleading error:
Parameter validation failed: Schema validation error: argument "content" is null

Treat null/blank input as "{}" before validation so that optional-field
schemas pass cleanly and required-field schemas still surface the expected
validation errors.

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