[ELF] ObjFile<ELFT>::initializeSymbols: don't call Allocate when firstGlobal==0

Calling `Allocate` with 0 size (when .symtab is absent, e.g.
`invalid/mips-invalid-options-descriptor.test`) may return a nullptr, which will
crash with -fsanitize=null (the underlying `Allocate` function is
LLVM_ATTRIBUTE_RETURNS_NONNULL).