Top level objects on a file with no contracts cause a KeyError: 'sourceMap' with Foundry

[elopez]

The JSON files generated have no sourceMap in them when there is only top-level objects; it might be related to foundry-rs/foundry#1646

Repo to reproduce: https://github.com/antoncoding/grappa @ 5886d166ee7b356ca1931c00fda63f7d6bc22b70

// SPDX-License-Identifier: MIT
pragma solidity =0.8.13;

uint8 constant UNIT_DECIMALS = 6;
uint256 constant UNIT = 10**UNIT_DECIMALS;
uint256 constant BPS = 10000;

Traceback (most recent call last):
  File "/slither/slither/__main__.py", line 755, in main_impl
    ) = process_all(filename, args, detector_classes, printer_classes)
  File "/slither/slither/__main__.py", line 76, in process_all
    compilations = compile_all(target, **vars(args))
  File "/usr/local/lib/python3.10/dist-packages/crytic_compile/crytic_compile.py", line 637, in compile_all
    compilations.append(CryticCompile(target, **kwargs))
  File "/usr/local/lib/python3.10/dist-packages/crytic_compile/crytic_compile.py", line 117, in __init__
    self._compile(**kwargs)
  File "/usr/local/lib/python3.10/dist-packages/crytic_compile/crytic_compile.py", line 548, in _compile
    self._platform.compile(self, **kwargs)
  File "/usr/local/lib/python3.10/dist-packages/crytic_compile/platform/foundry.py", line 143, in compile
    compilation_unit.srcmaps_init[contract_name] = target_loaded["bytecode"][
KeyError: 'sourceMap'
None
Error in .
Traceback (most recent call last):
  File "/slither/slither/__main__.py", line 755, in main_impl
    ) = process_all(filename, args, detector_classes, printer_classes)
  File "/slither/slither/__main__.py", line 76, in process_all
    compilations = compile_all(target, **vars(args))
  File "/usr/local/lib/python3.10/dist-packages/crytic_compile/crytic_compile.py", line 637, in compile_all
    compilations.append(CryticCompile(target, **kwargs))
  File "/usr/local/lib/python3.10/dist-packages/crytic_compile/crytic_compile.py", line 117, in __init__
    self._compile(**kwargs)
  File "/usr/local/lib/python3.10/dist-packages/crytic_compile/crytic_compile.py", line 548, in _compile
    self._platform.compile(self, **kwargs)
  File "/usr/local/lib/python3.10/dist-packages/crytic_compile/platform/foundry.py", line 143, in compile
    compilation_unit.srcmaps_init[contract_name] = target_loaded["bytecode"][
KeyError: 'sourceMap'