Patch `fruit` against "Null literal string" error

[kennyweiss]

@tomstitt reported a compiler error with fruit when using the XL compiler on LLNL's blueos clusters.

.../blt-ddd5a0c/thirdparty_builtin/fruit-3.4.1/fruit.f90, line 292.53: 1515-009 (E) Null literal string is not permitted.  A single blank is assumed.
.../blt-ddd5a0c/thirdparty_builtin/fruit-3.4.1/fruit.f90, line 1083.27: 1515-009 (E) Null literal string is not permitted.  A single blank is assumed.
.../blt-ddd5a0c/thirdparty_builtin/fruit-3.4.1/fruit.f90, line 1097.15: 1515-009 (E) Null literal string is not permitted.  A single blank is assumed.
.../blt-ddd5a0c/thirdparty_builtin/fruit-3.4.1/fruit.f90, line 1109.15: 1515-009 (E) Null literal string is not permitted.  A single blank is assumed.

The offending lines are of the form: https://github.com/LLNL/blt/blob/74d4e534dea3c8fa7bfa4c6f9d17eb37a1230c7e/thirdparty_builtin/fruit-3.4.1/fruit.f90#L292

According to @ltaylor16 :

The fix is to add the explicit blank.  var = "" should be var = " ".
Looks like this should be fixed in blt.  We're using version 3.4.1.  Version 3.4.3 is available but does not fix this particular problem.