Re-add null check for exclude rule

[fawind]

Before this PR

We removed this null-check in #1875 because intellij flags this as

Condition 'rule.getGroup() != null' is always 'true'.
Method 'getGroup' inherits annotation from package org.gradle.api.artifacts, thus 'non-null'.

However this does not seem to be true and an internal library failed to upgrade because rule.getGroup() returns null.

Stacktrace:

org.gradle.api.internal.tasks.TaskDependencyResolveException: Could not determine the dependencies of task ':checkUnusedDependenciesTest'.
Caused by: java.lang.NullPointerException: null value in entry: group=null
	at com.google.common.collect.CollectPreconditions.checkEntryNotNull(CollectPreconditions.java:32)
	at com.google.common.collect.ImmutableMap.entryOf(ImmutableMap.java:172)
	at com.google.common.collect.ImmutableMap$Builder.put(ImmutableMap.java:282)
	at com.palantir.baseline.plugins.BaselineExactDependencies.excludeRuleAsMap(BaselineExactDependencies.java:235)
	at com.palantir.baseline.plugins.BaselineExactDependencies.lambda$configureSourceSet$7(BaselineExactDependencies.java:173)
	at com.palantir.baseline.plugins.BaselineExactDependencies.lambda$configureSourceSet$8(BaselineExactDependencies.java:173)
	at org.gradle.internal.ImmutableActionSet$SetWithFewActions.execute(ImmutableActionSet.java:285)
       [...]

After this PR

==COMMIT_MSG==
Re-add null check for exclude rule
==COMMIT_MSG==

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Type

• Feature
• Improvement
• Fix
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• Deprecation
• Manual task
• Migration

Description

Re-add null check for exclude rule

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[svc-autorelease]

Released 4.19.0