optional string property in model does not work properly with "exists" where clause

## Steps to reproduce

create a model with an optional string property i.e.
` @property({
type: 'string',
})
c_modelCode?: string;`

when using "exists" on this property, doesn't return any data even when there is data.
`"where" : {
"c_modelCode": {"exists" : true}
}`

if the property is removed from the model definition, data is returned with the above where filter.

## Current Behavior

no data returned with "exists" clause even though data exists

## Expected Behavior

return all objects with c_modelCode propery.

## Link to reproduction sandbox

## Additional information

+-- @loopback/authentication@7.3.1
+-- @loopback/authorization@0.9.1
+-- @loopback/boot@3.4.1
+-- @loopback/context@3.17.0
+-- @loopback/core@2.16.1
+-- @loopback/openapi-v3@5.3.1
+-- @loopback/repository@3.7.0
+-- @loopback/rest@9.3.1
+-- @loopback/rest-explorer@3.3.1
+-- @loopback/service-proxy@3.2.1
+-- loopback-connector-mongodb@6.0.1

Start by reproducing the optional string property query against the listed loopback-connector-mongodb version, then trace how the repository handles the "exists" filter for declared model properties. Done means records containing c_modelCode are returned and a regression test covers the optional-property case.

Written by the indexing model from the issue text.