[SYCL] Fix -Wstring-concatenation in integration headers

[al42and]

A sentinel value added in #15175 fixed the problem of empty arrays, but for non-empty arrays was just concatenated to the last valid value. This produced warnings when compiling SYCL applications, and also meant that there is no sentinel value in the non-empty arrays.

[al42and]

Ping @premanandrao

[al42and]

@intel/llvm-gatekeepers, could you please start the CI?

[al42and]

@intel/llvm-gatekeepers, this PR seems ready, could you please merge?

[premanandrao]

@intel/llvm-gatekeepers, there is some urgency with this change as it needs to go into a product branch.