function _concatEtag(aResponses) {
		return aResponses.reduce(function(sCacheKey, oResponse) {
			// eslint-disable-next-line no-return-assign
			return oResponse.cacheKey ? sCacheKey += oResponse.cacheKey : sCacheKey;
		}, "") || null;
	}
Concatenates all Etag from a list of flex data request responses headers into a passed result string.

@param {object[]} aResponses List of responses containing Etag header to be concatenated
@param {string} [aResponses.etag] Etag value
@returns {string | null} Concatenated string of all etag values or null if no responses headers carry a etag value
@private