RFR: 8339068: [Linux] NPE: Cannot read field "firstFont" because "<local4>" is null

[Jose Pereda]

This PR adds a couple of null checks to `LogicalFont` and `FTFactory`, that make use of `FontConfigManager::getFontConfigFont`, which under certain cases, can return null.

I've tested this PR on both Linux (Ubuntu 22.04.4) and Android, just using `-Dprism.useFontConfig=false`.

On Ubuntu, I've manually added some font files to the JDK path `$JAVA_HOME/lib/fonts` (which didn't exist), and the test passes now, though this message is still printed out:

Error: JavaFX detected no fonts! Please refer to release notes for proper font configuration

which is confusing to me, because fonts where found in the JDK path after all, and even in the case that there were no fonts found, "the release notes" is an ambiguous reference for the user. 

Also, instead of adding fonts to the JDK, I tested adding a `logicalfonts.properties` file with `-Dprism.fontdir` and without `fontConfig`, but this case was already working before the patch for this PR, and still works after it.

Note that if there are no fonts in the JDK path, and `prism.fontdir` is not provided, without `fontConfig`, after this PR, there will still be an exception:


Caused by: java.lang.NullPointerException: Cannot invoke "com.sun.javafx.font.FontResource.getDefaultAAMode()" because the return value of "com.sun.javafx.font.LogicalFont.getSlot0Resource()" is null
        at javafx.graphics at 24-internal/com.sun.javafx.font.LogicalFont.getDefaultAAMode(LogicalFont.java:456)
        at javafx.graphics at 24-internal/com.sun.javafx.font.LogicalFont.getStrike(LogicalFont.java:461)
        at javafx.graphics at 24-internal/com.sun.javafx.font.PrismFont.getStrike(PrismFont.java:79)
        at javafx.graphics at 24-internal/com.sun.javafx.text.PrismTextLayout.setContent(PrismTextLayout.java:139)
        at javafx.graphics at 24-internal/javafx.scene.text.Text.getTextLayout(Text.java:303)
        at javafx.graphics at 24-internal/javafx.scene.text.Text.needsFullTextLayout(Text.java:258)
        at javafx.graphics at 24-internal/javafx.scene.text.Text$6.invalidated(Text.java:576)
 ...


because `LogicalFont::getSlot0Resource` will return null. Adding null checks after this point will require many changes (`LogicalFont::getSlotResource(0)` is used in many places). Shouldn't we fail gracefully after `LogicalFont::getSlot0Resource` if `slot0FontResource` is finally null?

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Commit messages:
 - prevent NPE if FontConfigManager.FcCompFont is null

Changes: https://git.openjdk.org/jfx/pull/1546/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1546&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339068
  Stats: 12 lines in 2 files changed: 6 ins; 2 del; 4 mod
  Patch: https://git.openjdk.org/jfx/pull/1546.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1546/head:pull/1546

PR: https://git.openjdk.org/jfx/pull/1546