Jib does not provide meaningful exception message jib:build

**Environment**:

- *Jib version:* 3.3.1
- *Build tool:* Maven:3.6
- *OS:* "linux", version: "2.6.32-754.41.5.el6.x86_64", arch: "amd64", family: "unix"

**Description of the issue**:
Got exception during jib:build plugin job, and exception does not represent meaningful output, since it says that map value is null, but does not specify the key
> labels map contains null values

**Expected behavior**:
Output message like this:
> labels map contains null values for key {//key name//}

**Steps to reproduce**:

1. mvn clean -U install

**`jib-maven-plugin` Configuration**:
```xml

com.google.cloud.tools
jib-maven-plugin




target/mosek
/deployments/mosek


../mosek
/deployments/mosek
mosek.lic




/deployments/mosek
755


/deployments/mosek/*
755





```

**Log output**: Failed to execute goal com.google.cloud.tools:jib-maven-plugin:3.3.1:build (default) on project tradefinance-optimizer: Execution default of goal com.google.cloud.tools:jib-maven-plugin:3.3.1:build failed: labels map contains null values

Full exception trace:

[16:09:19] : [Step 10/15] Caused by: java.lang.IllegalArgumentException: labels map contains null values
[16:09:19] : [Step 10/15] at com.google.common.base.Preconditions.checkArgument (Preconditions.java:145)
[16:09:19] : [Step 10/15] at com.google.cloud.tools.jib.configuration.ContainerConfiguration$Builder.setLabels (ContainerConfiguration.java:241)
[16:09:19] : [Step 10/15] at com.google.cloud.tools.jib.api.JibContainerBuilder.setLabels (JibContainerBuilder.java:462)
[16:09:19] : [Step 10/15] at com.google.cloud.tools.jib.plugins.common.PluginConfigurationProcessor.processCommonConfiguration (PluginConfigurationProcessor.java:441)
[16:09:19] : [Step 10/15] at com.google.cloud.tools.jib.plugins.common.PluginConfigurationProcessor.processCommonConfiguration (PluginConfigurationProcessor.java:490)
[16:09:19] : [Step 10/15] at com.google.cloud.tools.jib.plugins.common.PluginConfigurationProcessor.createJibBuildRunnerForRegistryImage (PluginConfigurationProcessor.java:309)
[16:09:19] : [Step 10/15] at com.google.cloud.tools.jib.maven.BuildImageMojo.execute (BuildImageMojo.java:110)
[16:09:19] : [Step 10/15] at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo (DefaultBuildPluginManager.java:137)
[16:09:19] : [Step 10/15] at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:210)
[16:09:19] : [Step 10/15] at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:156)
[16:09:19] : [Step 10/15] at org.apache.maven.lifecycle.internal.MojoExecutor.execute (MojoExecutor.java:148)
[16:09:19] : [Step 10/15] at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:117)
[16:09:19] : [Step 10/15] at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject (LifecycleModuleBuilder.java:81)
[16:09:19] : [Step 10/15] at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build (SingleThreadedBuilder.java:56)
[16:09:19] : [Step 10/15] at org.apache.maven.lifecycle.internal.LifecycleStarter.execute (LifecycleStarter.java:128)
[16:09:19] : [Step 10/15] at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:305)
[16:09:19] : [Step 10/15] at org.apache.maven.DefaultMaven.doExecute (DefaultMaven.java:192)
[16:09:19] : [Step 10/15] at org.apache.maven.DefaultMaven.execute (DefaultMaven.java:105)
[16:09:19] : [Step 10/15] at org.apache.maven.cli.MavenCli.execute (MavenCli.java:957)
[16:09:19] : [Step 10/15] at org.apache.maven.cli.MavenCli.doMain (MavenCli.java:289)
[16:09:19] : [Step 10/15] at org.apache.maven.cli.MavenCli.main (MavenCli.java:193)
[16:09:19] : [Step 10/15] at sun.reflect.NativeMethodAccessorImpl.invoke0 (Native Method)
[16:09:19] : [Step 10/15] at sun.reflect.NativeMethodAccessorImpl.invoke (NativeMethodAccessorImpl.java:62)
[16:09:19] : [Step 10/15] at sun.reflect.DelegatingMethodAccessorImpl.invoke (DelegatingMethodAccessorImpl.java:43)
[16:09:19] : [Step 10/15] at java.lang.reflect.Method.invoke (Method.java:498)
[16:09:19] : [Step 10/15] at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced (Launcher.java:282)
[16:09:19] : [Step 10/15] at org.codehaus.plexus.classworlds.launcher.Launcher.launch (Launcher.java:225)
[16:09:19] : [Step 10/15] at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode (Launcher.java:406)
[16:09:19] : [Step 10/15] at org.codehaus.plexus.classworlds.launcher.Launcher.main (Launcher.java:347)