Patch `fruit` against "Null literal string" error

Nobody has claimed this yet.

@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.
```

Open thirdparty_builtin/fruit-3.4.1/fruit.f90 and inspect the reported lines 292, 1083, 1097, and 1109. Check the empty string literals against the issue's compiler diagnostic, then verify that the XL compiler no longer reports the null literal string errors.】【。

Written by the indexing model from the issue text.