Substructure matcher finds implicit hydrogens as atoms

**Steps to Reproduce**

```
smiles = "C";
smarts = "[!#6]";
Indigo indigo = new Indigo();
IndigoObject pattern = indigo.loadSmarts(smarts);
IndigoObject molecule = indigo.loadMolecule(smiles);
IndigoObject match = indigo.substructureMatcher(molecule).match(pattern);
if (match !=null) // a match is produced here. Working correctly for version 1.12.
```

**Actual behavior**
Match returns non-zero result for implicit hydrogens

**Expected behavior**
There should be no match for implicit hydrogens

**Environment details:**
- 1.18

Start by reproducing the issue with the provided Indigo, loadSmarts, loadMolecule, and substructureMatcher calls using molecule "C" and pattern "[!#6]". Locate the substructure-matching entry point and verify that implicit hydrogens are not returned as atom matches; add or update a regression test if the project has one for this behavior.

Written by the indexing model from the issue text.