Fix binding options against null values

[bart-vmware]

Description

This PR fixes CompositeConfigurationProvider (a shared building block for the placeholder and decryption configuration providers) to properly handle null entries in configuration.

Fixes #1659.

Quality checklist

• Your code complies with our Coding Style.
• You've updated unit and/or integration tests for your change, where applicable.
• You've updated documentation for your change, where applicable.
  If your change affects other repositories, such as Documentation, Samples and/or MainSite, add linked PRs here.
• There's an open issue for the PR that you are making. If you'd like to propose a new feature or change, please open an issue to discuss the change or find an existing issue.
• You've added required license files and/or file headers (explaining where the code came from with proper attribution), where code is copied from StackOverflow, a blog, or OSS.

[github-actions]

Summary - All Code Coverage (ubuntu-latest)

Assembly	Line coverage	Branch coverage
Steeltoe.Bootstrap.AutoConfiguration	97.4%	100%
Steeltoe.Common	84.3%	77.8%
Steeltoe.Common.Certificates	96.2%	85.5%
Steeltoe.Common.Hosting	84%	70%
Steeltoe.Common.Http	100%	85.2%
Steeltoe.Common.Logging	81.1%	56.2%
Steeltoe.Common.Net	64.5%	66.6%
Steeltoe.Configuration.Abstractions	96.1%	90.7%
Steeltoe.Configuration.CloudFoundry	99.1%	91.8%
Steeltoe.Configuration.ConfigServer	97.2%	91.6%
Steeltoe.Configuration.Encryption	97.6%	92.4%
Steeltoe.Configuration.Kubernetes.ServiceBindings	95.1%	89.3%
Steeltoe.Configuration.Placeholder	93.8%	84.7%
Steeltoe.Configuration.RandomValue	93.2%	90%
Steeltoe.Configuration.SpringBoot	98.3%	95%
Steeltoe.Connectors	93.9%	89.4%
Steeltoe.Connectors.EntityFrameworkCore	81.5%	75%
Steeltoe.Discovery.Configuration	92.3%	100%
Steeltoe.Discovery.Consul	97.6%	96.5%
Steeltoe.Discovery.Eureka	92.2%	85.3%
Steeltoe.Discovery.HttpClients	94.3%	95.4%
Steeltoe.Logging.Abstractions	99.4%	96.9%
Steeltoe.Logging.DynamicConsole	100%	95.4%
Steeltoe.Logging.DynamicSerilog	99.1%	95.4%
Steeltoe.Management.Abstractions	100%	100%
Steeltoe.Management.Endpoint	95.8%	89%
Steeltoe.Management.Prometheus	95.8%	91.6%
Steeltoe.Management.Tasks	100%	****
Steeltoe.Management.Tracing	100%	75%
Steeltoe.Security.Authentication.JwtBearer	100%	100%
Steeltoe.Security.Authentication.OpenIdConnect	73.8%	59%
Steeltoe.Security.Authorization.Certificate	96.7%	75%
Steeltoe.Security.DataProtection.Redis	100%	****

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
76.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[TimHess]

LGTM