[isoltest] Allow empty input (even when expected) in TestFileParser

Actually the test parser doesn't allow empty input. This fails parsing:
```
// deposit(bytes,bytes,bytes,bytes32), 32 ether: -> FAILURE # Empty input #
```

With:
```
semanticTests/externalContracts/deposit_contract.sol: Exception during test: Throw location unknown (consider using BOOST_THROW_EXCEPTION)
Dynamic exception type: solidity::frontend::test::TestParserError
std::exception::what: Line 188: No argument provided.
[solidity::util::tag_comment*] = Line 188: No argument provided.
```

_Originally posted by @axic in https://github.com/ethereum/solidity/pull/10590#r542372647_

Locate TestFileParser and reproduce the failure using semanticTests/externalContracts/deposit_contract.sol, focusing on the empty input in the shown comment. The work is done when this input parses without the Line 188 “No argument provided” error while existing parser behavior remains intact.

Written by the indexing model from the issue text.