Address control reaches end of non-void function warning
Address control reaches end of non-void function warning

firestore/src/common/field_value.cc: In member function ‘std::string firebase::firestore::FieldValue::ToString() const’:
firestore/src/common/field_value.cc:341:1: warning: control reaches end of non-void function [-Wreturn-type]
  341 | }

PiperOrigin-RevId: 299217768

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