182 erroneous read only on server start

[StevenRojasC]

This PR fixes an unexpected query response containing an exception message when running a Query Edge/Node transaction on a new/empty PMGD db instance.

Closes #182
Closes #307

[github-actions]

Target CPP Coverage: 71.4335%
Source CPP Coverage: 71.3273%

Target Python Coverage: 97.94%
Source Python Coverage: 97.94%

[github-actions]

Target CPP Coverage: 71.4335%
Source CPP Coverage: 71.3273%

Target Python Coverage: 97.94%
Source Python Coverage: 97.94%

[StevenRojasC]

Since the code coverage was reduced by more than 0.1% with these changes, the following screenshot summarizes the line numbers that tests are covering (new one on the left, original on the right):

Here's some references to the code lines that are not been executed by the tests:
PMGDQuery.cc (89-92):

vdms/src/PMGDQuery.cc

Line 89 in 6ce7c3a

if (_pmgd_responses.size() == 1 && _pmgd_responses[0].size() == 1) {

PMGDQueryHandler.cc (200-203):

vdms/src/PMGDQueryHandler.cc

Line 200 in 6ce7c3a

} catch (Exception e) {

QueryHandlerPMGD.cc (368-383):

vdms/src/QueryHandlerPMGD.cc

Line 368 in 6ce7c3a

Json::StyledWriter writer;

As you can notice, all of the code that is no longer being executed by the tests is error handling code. This code was being executed before due to the exception thrown at https://github.com/omp87/pmgd/blob/f52a0b3c2c661bb07ae3218214557165f72a0b22/src/TransactionImpl.h#L189, which is exactly what this PR tries to fix, even though we don't have control over the pmgd codebase. Since the problem is fixed and this exception is never being reached, we would expect the error handling code to never run.

This is how the output would look like when this error comes up.