Respect '' received from aquery

GoCompilePkg action's aquery results contains ''. e.g (the interesting
ones)
```
/bin/bulder ... -embedroot '' -gcflags '' -asmflags ''
```

strings.Join would cause it to appear as a whitespace and cause issues.
(in ninja without this CL)
```
/bin/builder ... -embedroot  -gcflags  -asmflags
```

Convert the empty literal to '' before strings.Join

To limit inadvertent side-effects, this limits it to GoCompilePkg mnemonic

Test: TH
Change-Id: I67161c194dabac6f857ff49b85d4b2471970a9b2